I nerfed the field of sealing to the point that it actually made sense with the context of the show and how unpopular it is as a tool. If it really were as effective as fandom portrays then it would probably be as emphasized as at least puppetry or medical jutsu. And, if fuinjutsu were that powerful then there would be no reason for the OC to not exploit it, which would give him too many OP styles to choose from, resulting in a curb stomp that wouldn't even have time for a build up. Which would be kind of boring.
It
is as effective as fanfiction portrays, it's just that most people in Naruto don't know of it. Plus, fanfiction gets a few things wrong in their portrayal of fuinjutsu, but not about fuinjutsu itself. They mess up by making themselves or their chosen chracter God-like at making fuinjutsu, when even the 4th Hokage made
one fuinjutsu of his own(the Hirashin) and adapted, with Kushina's help, made a superior seal by combining existing Uzumaki techniques(Naruto's Reaper Death Seal), and
he was considered a prodigy at fuinjutsu. They make themselves able to make dozens of fuinjutsus based off of ideas from shows or movies or comics from their old lives, when they really shouldn't be able to. I doubt they all have perfect handwriting and the time and patience to learn fuinjutsu when trying to be a ninja or that every project they have goes perfectly. But they never show the time and patience it takes to learn or make
one fuin or the consequences of messing up(whether big or small). Fuinjutsu
is overpowered(and rightfully so), but the writers mess up by making
themselves so good at it and then churning out the fuins with no problem.
The reason why puppetry and medical ninjutsu are more well known is because they actually have
less exacting standards(basically all you need is perfect or close to perfect chakra control and you're good to learn) and have more teachers in the subject/there's more knowledge about them or the subject available and they promise more
immediate results than Fuinjutsu. Plus, one of them is the specialty of Suna and the other is universally useful for all ninja villages.
As for the curbstomp issue, it wouldn't be an issue for a
long time or ever really. The SI/OC could not have the perfect handwriting necessary for Fuinjutsu or the time available and patience needed for it. The SI/OC could be afraid of the potential disasters of messing up a fuin which puts him off it. The SI/OC could know Fuinjutsu takes a long time to learn and master(let alone decode anothers' work-otherwise Jiraiya would've learned the Hirashin long after Minato died) and prefer to go after Nin/Gen/Taijutsu or he just likes Nin/Gen/Taijutsu.
Plus, even if the SI/OC was to go after fuinjutsu, it would take
years to make some OP fuin to curbstomp his enemies with. Even if he had something to base his work off of(say for example he was a Konoha-nin, I doubt they just give him one of Minato's Hirashin kunai to try and decode and Jiraiya himself couldn't figure it out), it'd take years to decode it and then make sure he could do it properly.
See below for other reasoning.
Honestly, I am completely unsurprised with how unpopular it is in the show even if it was a method of becoming ninja jesus. Remember that ninja typically graduate at 12 or earlier. Sealing itself is a precise art requiring much patience and knowledge to make use of. Once they are out of the academy they need to support themselves with missions so that is a significant chunk of each day taken up with doing a D-rank and training with their team. At this point they aren't likely to even desire the rather obscure art. later on when doing multi-day C-ranks they have even less time to study and most of their free time is likely spent doing things that provide a much more immediate benefit like physical training or studying in their specialty if it's nin/genjutsu. And we see in series that the benefits of those methods never really wain. I mean, shit, they typical ninja gets by on how many unique techniques?
Basically, this. It's
so much easier to simply learn a few katas to a martial art/learn to throw kunai and swing a sword and exercise(hard) or practice your hand signs and learn a new ninjutsu or genjutsu than to sit down and learn fuinjutsu.
As I said in Diomedon's Quest thread
Staying Frosty: "Well the thing is, fuinjutsu by it's very nature
is incredibly overpowered and bullshit. Otherwise humanity never would've been able to seal away the bijuu and Kurama would've killed Hashirama and Mito and basically everyone in the planet after Madara lost at the Valley of The End/Hashirama wouldn't have gone after the rest of the Bijuu and given them away to the rest of the villages(fucking stupid idea) or Madara would've won because Mito didn't manage to seal Kurama at the Valley of The End or any of the bjuu could've just decided to exterminate humanity for whatever reason(some very stupid human decided to poke them with a stick for example).
The only reason why most people don't take advantage of it or why it's not very well known or widespread is due to several factors. Time, patience, the need for
perfect handwriting, the lack of teachers(and the fact that Ushio is/was destroyed and the Uzumaki are near extinct/scattered), the need to learn the thousand and one different brushstrokes and words(kanji/katakana/etc...) and what they mean/lead up to/build together, the need to have the creativity or ingenuity to create your own(Journeyman and above) fuinjutsu, etc... Most ninja never go for it because it's so much
easier to simply learn a few kata for taijutsu and be able to beat your enemies into submission or handsigns and be able to make your enemy see their darkest fears or blow a great ball of fire and burn your enemies, and some ninja might never even learn about it because it doesn't seem like something the Ninja Academy teaches about or mentions(Due to the fact that excitable kids trying out fuinjutsu without supervision or even learning of it and shit is a
Bad Idea)."
Those are some of the reasons why fuinjutsu isn't that widespread and the most important ones. Plus, unless you are a seal master(and perhaps not even then-depending on what you focus on in fuinjutsu), you can't make your fuin in the middle of battle(as you need time and a steady hand to not fuck up the complicated fuin), you'd have to prepare them beforehand and even then you cannot prepare for
every eventuality.
Also, as Diomedon said: "There are limits which you might learn about if you pursue it, and the most complex arrays and seals can fail in more catastrophic and interesting ways than simple explosions, which discourages experimentation in general. A ninjutsu specialist might invent (or in many cases reinvent) half a dozen jutsu over their career, and perhaps one of those would actually be above C rank (and then it would usually become their signature move). A fuinjutsu specialist might make half a dozen innovative alterations to existing arrays to increase effectiveness over their carrer, and if they are particularly talented they might attempt a single new array. The 4th Hokage made one innovative array, and with Kushina's help made a superior seal by combining existing Uzumaki techniques."
That also hurts the field of fuinjutsu, because you can't rush making or testing fuins without potentially disastrous consequences and very little new techniques might be made(unless you are a fucking Uzumaki) and most of the knowledge isn't widely shared amongst the fuinjutsu community and with the fall/destruction of Ushiogakure, much knowledge was lost and even more gets lost as less and less is passed on/more teachers die without finding a student. So knowledge has advanced slowly, because it isn't like the villages are willing share fuinjutsu techniques, and due to it paying off late in a shinobi's career, it's a rare specialization in the first place. Why train something that wont pay off for years when you're worried about dying next week or next month?
Tl;dr: Fuinjutsu
is overpowered by nature(otherwise there would be no jinchuurikis/ the Bijuus could never be sealed). It just has several problems regarding it. The requirements to learn fuinjutsu are really strict and exacting, not much progress is made in the field due to fear of potentially disastrous consequences(such as dying or wiping out the entire continent-it varies) and knowledge advances slowly as a result, it pays late and most shinobi won't train something that wont pay off for years when they are worried about dying next week or next month, and it's not well known by the greater community because of it.
