I don't think there will be issues. Monty hooks into Post actions so with bbPress I guess nothing will happen.
The main issues (which need to be addressed) are things like when Monty asks you to check something. Especially with new users on potential Splogs if you have set Monty to check every first blog post (not comment) Monty will tend to ask you to check all the posts of that user until you do that first check. If you are getting a lot of people signing up this can be a little annoying, and this is the kind of interface issue that needs to be improved.
Also, there seems to be a small bug whereby Monty doesn't automatically spam the signup data if you mark the blog post as spam - I've just noticed this. It means an extra step of doing this manually - I'll fix it in due course.
On the other hand, stopping comment spam works almost perfectly in my experience, and since on my installs this is a far bigger problem, this has been great.
For comment spam (see below) we have 12,000 comment blocks, 10 checks by me, 0 errors ( I got lucky there - Monty usually makes one or two errors to begin with). The interesting thing is that he has only 22 spams in his database meaning that most spams are fairly similar. It's interesting to see the techniques that spammers try to use. My favourite is the random word stuff:
tnhhn nthuyknth ypri nhuou ntheus
When Monty sees this he goes, "Ah, 5 words I don't know". Rate each 50%, overall rating 50%, but there may be other issues that suggest the post is spam. But since the poster has a spammy rating of 50% he hasn't hit the magical 10% that he needs anyway. Monty will tell you to check this, and when you do he gets the other header data that the spammer sent. So these random words just help him because they teach him the patterns that spammers use.
Stats
Total Number of Spam Block
12702 accesses. Oh Yeah!
Straight Block
10610 accesses. Yeah!
Comment Spam
Token Database
Spams: 22 messages with a total of 2412 tokens
Hams: 13 messages with a total of 1265 tokens
Overall
Spam Stopped:2075
Ham Let Through:13
Classified as Unknown
Spam:6
Ham:4
These are messages that Monty needed help with. Will be higher in the early stages.
Errors
Wrongly spammed:0
Wrongly hammed:0