I think that in general Assassin is pretty good, and I think Jack is a lot different than True Assassin/Old Man of the Mountain. She is definitely still a master targeting ambush fighter, but her skills make her more capable of evading an enemy servant long enough to get to the master. Assassin relies on the multiple bodies trick to distract people and move focus, but is a bit lacking in direct offensive power without over-concentrating bodies and becoming vulnerable to powerful large scale noble phantasms. Jack carries more power as an individual, but is more vulnerable to anti-personal attacks, as well as to trickery on the opposite side
I honestly thought that Assassin was poorly used in both the wars we see, but in part that may be due to him being handicapped by Kirei's alliance with Tohsaka in the fourth war, meaning he was supposed to mostly hide and play passively, until he got ordered into the shittiest situation ever against 3 incredibly powerful servants.
In the fifth I have am honestly still confused what Zouken was trying to do, all I know is he was a creepy man made of worms that needed to die yesterday. In fire.
TLDR: I think Assassin got shafted by his master in both wars we see, and should get a bit more credit. As a final thought, I think assassin's usefulness may depend more on the master's skills to a greater degree than another servants. But I've said to much, i'll theorize ore if people want, but or now i'll shut up.