Scott Bakula isn't a bad actor... after all, he had earned himself a Golden Globe for Best Actor win, two additional Golden Globe nominations as well as four Emmy nominations for his role in Quantum Leap. Recently, he was also nominated as Best Supporting Actor for his role in Steven Soderbergh's Behind the Candelabra.
Well I for one am not saying he is a "bad actor". He did fine in the roles you mention. He was just ill suited to play Archer. A better actor (in the role) might have overcome some of the weaker writing. As I have written, better acting overcomes bad writing all the time.
The problem was, Scott had to have
good writing in order to shine as Archer and couldn't rise above when he didn't get it. The same writers who wrote Archer also wrote Trip and Phloxx (and everyone else). But we saw Trinneer and Billingsley defeat bad characterizations with their natural, relaxed, and charismatic performances.
This is the reason Scott had all those meetings with TPTB (think it was after season 2). Just because your character is written as a jackass does not mean the role cannot be payed in a way that causes the audience to like or respect the character despite it being obvious the character is a jackass. Scott couldn't do that with Archer.
BTW, I believe that both the Quantum Leap and Candelabra roles were right in Scott's wheelwell. When he is given the "perfect" role (for him), he does fine. But give him somthing that is a bit outside of his normal "shooting range", and he "bricks it" (to use some basketball analogies) -- Archer.