Well, you are locked into a particular implementation of the rewrite. If the developer decides to drop the project you will either need to update it yourself or pay someone to do so. If you drop to normal URLs, you lose all rank and backlinks for those pages. (Of course, dropping support is a problem with all software.) In addition, depending on how the feature works, it might cause a bit more load on the system. This won't be bad while the forum is small, but if it grows, you'll now have to contend with possible overhead that limits your scalability, causing you to need to upgrade sooner than you might have had to otherwise. (The impact really depends on many factors, so it might be greater or less. I haven't benchmarked it.) In at least one case, it was found that you were penalized for keywords if you happened to use very competitive terms in your URLs. This study is a bit old, and it might not be the case anymore. I don't think the situation would be reversed, however. At best, the impact is now neutral.