The problem with Ikonboard is that it is very resource intensive which can cause havok. Especially when multiple people are running it at once. This pushes the server load very high which is unfortunately not acceptable. The problem is not the database part, it's the script that is killing the server, eating resources. It looks good on the surface but behind it's bad coding - very sloppy.
As for YaBB it's only the flat file version that is banned. YaBB SE (the PHP/MYSQL version) is ok.