in the late eras they are terrible:
no hide ability
no reactive armory
no ability to hit hidden units without being next to them
no ability to shot back when hit
not being able to hit more than one unit at the same time
(did I miss a good ability of any unit)
and most important:
costs you points when you attack with them. because it has the highest points so you will also lose more points for any damage they take
I don't think a unit's point value is 'most important' when discussing army viability, to be fair. Rail Guns for example are worth almost as many points as Champions are but that isn't stopping anyone from using those (if it was the 'most important' factor then that would definitely be a consideration for their use). Mostly nitpicking here over your word choice, but yeah.
Anyway, your assessment depends on which eras you're talking about.
-PME they're great. Use them instead of MGs or whenever there's a lot of artillery on the other side (MGs hate artillery).
-CE they're also solid, but generally not needed because of how good AAs are and how bad the artillery is (otherwise would be a similar situation to PME)
-TE they're the best unit in the age. They'll kill anything in 2 hits and can hit APs from out of range, both things the Stealth Tank can't do (or can't do as easily). With their massive stat advantage, they don't even need the stealth.
-FE they suffer a bit because of how strong Rail Guns are, but they're still a better option in any situation where you don't want to use (all) RGs.
-AF is probably their weakest age. They can still hit hard, but Battle Fortresses are a significant power improvement over Hover Tanks so the gap's much smaller and the bump in Reactive Armor is huge. They also have more range and mobility.
-OF they're about as good as Crabs...take that as you will. They have more trouble against Eels and can't hit Subs as hard but they can outrange Mantas. Of course, all of this is referring to battles where you aren't going to just use Turtles/Hovers anyway.
Champions do fall off near the end, but once they become heavy units they stay relevant for a pretty long time (try to have and keep one for each age starting with PME) and they always have the highest or near-highest stats in their age group.
