The big difference between flat-file and database backed forum systems is that the load for accessing the file system and doing all the nasty bits get shifted to the database server. There are several other things that make a database backend better, but it is possible to run large boards on a flat-file based system without many problems.

I won't lie: in many cases database backed systems perform better; however, it doesn't really matter unless you have a very popular board with lots of posts, activity, etc.

but it is possible to run large boards on a flat-file based system without many problems

umm... no. flat file based stuff is not scalable because most (all?) of the software using this kind of db, does not implement a sophisticated filesystem. Also there is a possibility to reach a # of files in a directory limit in shared hostings ith too many members etc. A database removes these kind of problems. Even if you have your own vps/machine and use a flat file system on a very busy site, you'll experience huge slowdowns related to too much disk access. Even DB_File will be better than a hand crafted flat file db.

in many cases database backed systems perform better

I think it's better in every case unless you have only 10 users.

I think you are underestimating flat-file boards. I know that the YaBB 2 development team spent an incredible amount of time working to speed the thing up, and have done a very good job.

Database systems still have to access the disk at some point. They still store files on the hard drive. There are obvious differences with how they work, but it is not fair to just make a blanket statement that software based on a flat-file system is not scalable. Furthermore, assuming that sophisticated techniques are required to speed things up is preposterous. I don't think I've seen a single YaBB user complain about having too many files in a single directory.