I think folks tend to confuse how good the actors performed with how their parts are written.
For TNG, Stewart (obviously) was very good - but if you remember, it too a little while for Patrick to be comfortable on TV. He too was at home on the stage.
Frakes was good from the start and probably the most consistent from start to finish.
Spiner should have won awards for his performances as Data. Truly great.
All the rest were just fine, with the only real head scratcher being Lavar when given any type of romantic stuff. I don't know if it was the visor or what

but they weren't his best performances.
For DS9...
The ONLY way this series stayed on the air is that the supporting characters/actors, Odo, Quark, Garak, Obrien, etc were exceptional.
Avery Brooks is a horrible actor by any standard WRT to acting. He can't convey ANY emotion without overacting. He can't even get the voice inflections right. He over pronounces words all the time. I to this day can't believe they couldn't find a better black male actor. If this series had been on a star ship where he was at the center of the stories more often, I believe the show doesn't make it past the 1st season. To compare his acting skills to Stewart's is ridiculous.
Alexander Siddig was horrible in the 1st season, but at least he did seem to get it right starting with season 2.
Nana Visitor, as with many trek actors over the years across all series, overacted quite frequently early on (Torres, Stewart, Burton, Dorn, Keating even Mulgrew for a period comes to mind). But she too got better with experience and time. The same with Terry Farrell, but I attribute her improvement to a change in writing the character. Terry wasn't very suited to the perfect princess characterization they began with.
In short, if I watch the show and it looks like they are acting, they aren't doing their job. I saw that with DS9 more than any trek series.
IMO and the writers, the best crew of actors in the spin-offs was on Enterprise.