I wouldn't say Scott is a "bad" actor, I just think he was totally miscast as "Archer". The captain role in any Trek series is the most important and requires, according to most fans' tastes, at the least, presence and a certain amount of charisma. Scott was short on both.
Great acting can trump bad writing. We saw this numerous times in TOS, TNG, and DS9. We pretty much never saw it in Ent. Scott never seemed to be able to drag "Archer" up out of the muck of bad dialogue and characterization to the point that everyone at least respected the character even though he was being made to look ridiculous or contemptable. The entire show suffered mightily because of this.
As I have stated here before, it is no coincidence that the two least liked and respected of the Trek series' also have the least liked and least respected captain characters.
I certainly don't hate or even dislike Scott, he seems like a great person from what little I know about him, but he was as wrong as wrong can be for the role of Archer.