Pretty much this.
They certainly don't really live up to player expectations. D&D 5E is a game full of total badboy badasses, it really is. Berserker Barbarian really hit that archetype with a brick. Battlemaster Fighters do actually feel pretty masterful so long as they get them short rests. Bladepact Warlocks will chop you the hell up with a demon sword. Enchanter Wizards WILL control your mind. Loremaster Bards absolutely feel like they are. Even Diviners are badasses, which has absolutely never happened in D&D before.
And so on.
So any subclass that doesn't really whack that archetypical vibe is a bit meh. And that's Assassin. People expect a D&D take on like an Assassin's Creed-type badass (and really that's 99% of modern assassins across all media - acrobatic, dangerous badasses who spring out of hiding/drop from the rafters and kill a bunch of people before making some kind of wild ecape). What we get instead is someone takes the Hitman Codename 47 approach, except way more toned-down and weak (even ol' disguise-oriented 47 can go on a hell of a rampage when stuff gets real). They're not terrible, but they're just not like "HELL YEAH" like a lot of subclasses can be.