Well, if you face an AI superstack of them at a higher difficulty you might understand why some people complain. (had one that once around Turn 200 (deity level) against svarts once and henceforth i could at least grasp what people were complaining about. Even though i still disagree with it being anything of a serious problem...)
"Problem" is that they really obliterate smaller stacks (even of rather powerful units) if massed since they
a) are immune to first strikes,
b) have one themselves (or 2 in case of leaders with or the Sidar),
c) they poison units they fight + damage (and they do damage most units they fight) making them weaker still and hugely inflates the time they take to heal...,
d) kill units that get damaged rather fast since they attack them with priority (which they consistently do since of having more strikes in any case save attacking first strike immune units themselves in which case they are on par. Also that makes drill / first strikes very powerful for Assasins and especially Shadows / more powerful recons with marksmen promotion)
e) they level rather easily / safely + fast since they usually fight weaker targets with priority and thus get more kills on average on offense (which yields much more XP) than most other units... (this is much more important for non-reload games and especially AI than it is for a usual player in an everyday game...)
Those things combined work out in a rather nasty way (especially for Svarts obviously).
But i think its fine if the AI at least gets one tool it can use to devastating effect and that's not civ-specific (its not the only one but there are not that! many...).
In fact that imo is! something that strongly! speaks for! assasins in their current form (and having at least some AI prioritize poisons a bit more might improve it quite a bit...
) not against them...
As a player its much harder to mass them to that degree (even though its easier to combine them with other decent butt-kickers using them more effectively... but the AI needn't care for that part...
)
If massed and/ of highly experienced it really is an outstanding unit for its time and place in the game... (but that is true to a degree for the recon-line as a whole.
Especially for combat-oriented leaders or civs with a strong recon-line. Most notably Svartalfar and Sidar of course or for civs running FoL (or both.
)...
And assassins wouldn't be half as scary if they wouldn't offer so much synergy with Rangers + Satyrs if combined in one bick pack of butt-kicking goodness... Since on their own they really suck at attacking cities ... unless those are defended in a woefully inadequate way. Or to a lesser degree on defense.)
But the whole thing is only true if be-lined to a certain extent. But quite some units of that tier are powerful if be-lined (champions and mages spring to mind without much delay...) so imo the "all things overpowered leads to some way of balanced" works out sorts of ok here...
(onedreamer might disagree with that statement.
)
If you want some quick and dirty fix of firsthand experience, fire up a game with unrestricted leaders on immortal / deity (or monarch/emperor if you don't usually play immortal or above) with 8 players on a normal erebus map with low sea-level
and pre-assign (so no random leader) the 7 AI as follows:
Fearyl of Svarts
Sandalphon of Sidar
Tasunke of Svarts
Amelanchier of Svarts
Hannah of Svarts
Mahla of Svarts
Decius of Svarts (or Falamar/Sabatiel of Sidar once the AI gets taught how to wane units and settle shades in a meaningful way)
then you are sure to get some firsthand experience about fighting large numbers of assasins (the weights are still done for leaders but i belive the above cast should at least prioritize and offer some synergy with poisons...)