This appears to be a time for disappointing sequels, although for awhile there, we got a lot of top tier extremely competent sequels. I guess no trend can be permanent.
The first Boondock Saints was one of those rare creations that had just about the optimal amount of everything, it was balanced between being believable, ridiculous, funny, and brutal.
Balanced is the last word I would use to describe the sequel. The dialogue is terrible, just about everyone in the movie talks like a middle school bully. There are honest to goodness slapstick comedy moments, such as a mafia liutenant getting smacked in the face with a salami, and then a follow up seen where he's forced to wear headgear and can't speak properly.
The tone of the entire movie is just so very different from the original, that it feels like it was made with a different director/writer, with a different vision for what the movie should be. All the more sad, since it's the same writer/director, Troy Duffy, who's been working on getting this movie made for the last ten years.
The movie's overly long at a full two hours, and while it's not terrible, it does more to detract from the first movie than to bring anything new to the world of the Saints. It just constantly feels like Duffy is trying to imitate himself, which just breaks my brain.
If you really loved the first movie, I would recommend that you skip the sequel, and just rewatch the first one a couple more times.