I always found the commercial webhost avenue to be a source of frustration when dealing with the peculiar requirements of a high-traffic phpBB board, which was the primary reason I went with self-hosting. It's just too easy for a phpBB board to vacuum up all the resources of a webserver, and the nature of the software (particularly the stock search system, which I modified for speed) is such that without very generous RAM allotments, a growing board will quickly slow to a crawl.
Even with a modified search system, the size of the searchtable indexes alone tends to consume a lot of memory, and most webhosts configure their MySQL daemons to have relatively low memory usage. Despite the low cost of RAM nowadays, most webhosts tend to have surprisingly little RAM installed on their webservers.
So the result is that with a typical shared-hosting or VPS hosting solution, I get far more disk space than I need, but a small fraction of the RAM I need, even for a midsize board (my own board has long since grown beyond the point where a shared or VPS solution was even vaguely practical).