Well the problem in my opinion is that UBB is great for small to medium sized boards but once it grows large it chokes even on a large dedicated server (been there done that). The PhP accelerator helps but as noted really causes problems with modifications. VB's combination of PhP and MySQL is faster and is scalable with less system resources needed.
The only reason I did not switch is I felt that we could do a better job customizing the systems code vs how the other bulletin boards were coded since we did not have to cater to a market but rather only our system. I did look at all the other offerings and was made offers by more then 1 to make the switch.
While I see IPs reasoning behind not going with anything other then the pure basic server requirements I have to question how this will fare out in the future as PhP, MySQL and others are redily available, require little work to add to the server and will reduce over head tremendously. Wasn't it Bill Gates who exclaimed once that we will never need more then 640k?