Right so you've just got a tribe of fel orcs, and at some point sooner rather than later you're going to have to deal with it in a leadership position.
The Orcs first encountered the Fel when Gul'dan/Ner'zhul fooled the Orcs into thinking Kil'jaeden was a powerful ancestral spirit who had a new type of magic for everyone.
There are a variety of ways the Fel can be administered, but essentially it makes the user physically more powerful at the expense of mental instability. To some, this isn't a big deal, but the Orcs over time clearly understand that the Fel has negative effects to the wider environment, as well to what we might describe as 'public health'.
Like any social issue, there seem to be a number of ways to describe people's opinions on the Fel. These stem from the spiritual or cultural views of the individuals. For example, lots of the southern tribes, notably the Bleeding Hollow, Bonechewer and Laughing Skull, have a hostile relationship with their natural environment anyway, so I don't think they'd see a difference between the Evergrowth killing their guys vs the Fel poisoning things.
Comparably, with the exception of the Frostwolves, all the other clans are also fine with dominating their environment to a greater or lesser extent, for example the Blackrock are frequent users of Dark Shamanism and the Dragonmaw and known for taming creatures.
I think that most Orcs would be familiar with Gul'dan's trickery, and the idea that the Fel isn't actually that great. The issue though is that the Fel is undeniably powerful, and you might just forget at some point that they were doing something bad because it's just that useful. Even orcs like Blackhand when they felt the Fel had failed them in military affairs, didn't seem to have an inherent stance against it, it was merely a tactic or group of people who'd failed.
Doomhammer comes in, but there then emerges a question over the next few decades about whether the Fel is inherently bad, or whether it's something that stems from something bad, and is actually ok. It massively depends on the person's position in society, what they might think about Gul'dan for example. If you're one of the younger generation who don't remember Draenor, and didn't see the worst of the Fel-orcs under Grom for example, you might feel that the Fel is just an especially destructive magic. What fundamentally is the difference between a troll witch-doctor who sacrifices a prisoner and uses magic against an enemy, when compared with a warlock who also uses sacrificial magics to fight?
I think essentially the Fel and people's perception of it would be a matter of considering your individual position. Even Thrall, who had a pretty good reason to distrust the Fel, is ok with warlocks existing. If you do take a position that some amount of Fel is acceptable, you've already gone far enough that you're then just negotiating about how much. Unless you take an absolute position like the Kaldorei did, you have to accept a certain amount of corruption.
Now as to things like Fel-Orcs, can we assume that all the Warsong ones were killed and only Grom was cured? Or was there some sort of purification ritual for them too? Perhaps, but the Fel would have probably been fairly difficult to cure if it took loads of work, so maybe the Fel-orcs just all got killed.
In any case, there will be some Fel-orcs in the Horde, even in Thrall's Horde. They may be disliked, even disenfranchised, but they remain some of the most physically powerful warriors in the Horde, and the Horde still needs them as shock troopers. If you can throw some Fel-orcs at the problem instead of sending your own warriors, that's advantageous. In some clans, perhaps Blackrock, certainly Burning Blade, Fel-orcs are more accepted, but they're still not at the forefront, which is partly why it's so unusual that Grok walks about with a Fel-orc in such a prominent position as his trainer/adviser/guard.
I think that in many cases, this is another example of Thrall's inability to make a firm decision. He's caught in between wanting to control warlocks and use their formidable abilities, with the great utility Feldad brings for example, and his hatred of them due to their representation of the betrayal of his people. Someone like Feldad, who goes about pretty openly talking about the Fel, and who so famously won against Forneus while Thrall was unable to do anything, is an embarrassment for him.
On Grok taking on the Demonsword Clan, this is partly tied up with the military implications. Grok is already dangerous to Thrall, having thousands more warlike orcs is even more dangerous. The Fel-orcs themselves, or the Demonsword's reliance on Fel generally, is indeed gravely concerning though, and will continue to be so over time. Firstly, because if they can be controlled it demonstrates that they're pretty cool actually which really impact's Thrall's policies saying the Fel is bad. If they can't be controlled the implication is that Grok might be just as bad as the worst of his clan. There are other implciations, but those are perhaps the main ones.
On Grok's personal stance, this is an extremely complicated matter. Grok personally doesn't have anything against the Fel. Feldad has been very careful to not expose him to the bad sides of the Fel, and has carefully controlled who meets him, eg he sent Sesk and Ishi to demonstrate that Sesk is actually a controlled guy even when he has Fel stuff. Similarly, though Feldad demonstrates signs of physical corruption which might be unsightly, that's not really a big deal. In terms of Grok's personal stance, he's refused the Fel before, but he personally isn't necessarily opposed to it, he just personally hasn't had it. Grok has a general dissaproval of the Fel because of his personal beliefs, but that doesn't necessarily extend into a political stance. In terms of his official stance, Grok would be presently unable to denounce his father, due to the extreme responsibilities of leadership and personal attitudes Grok has toward Feldad, and he can't therefore lose 'face' by criticing the Fel. He takes a stance like with teh warlocks, by saying that he personally doesn't subscribe to his father's ideals, but still respects his father as clan chief, and obviously a parent.
I don't actually recall what prompted this post so have probably forgotten why I was writing it, just remembered that I was indeed going to remark upon it. If anyone has questions add them and I'll edit the post.