I agree with you. I'm watching Star Trek Enterprise for the second time and find that his acting is very stiff and it seems like he is yelling his lines. I think he does so well because he is very likable and relatable and this is what makes him an actor. Otherwise, he would not of had a career.

He was pretty great in 'The Informant!'. One particular scene with his FBI agent character and Matt Damon's character - the feelings scene - is just hilarious, but still warm and weirdly sweet. I like him in NCIS New Orleans, think he's pretty underrated as an actor and he makes that show for me.
"That's TV, Sammy. That's what you watch when you've got no brains like you and me."

You should see him in Quantum Leap which I grew up with and know him from. He was great in that and u might really see his true acting from the series.

I suspect you haven't seen enough of his work. I think he is an outstanding actor. It was the power and versatility of his performance in Enterprise and then Quantum Leap that made me a fan.