I'm going to chime in here. I'm new to the phpBB community but I've been around some others for a very long time (in MyBB and otherwise). Been running forums for many years and phpBB is an awesome software, glad to have finally joined here.
What others have said before me is absolutely correct. Unfortunately, the *main* thing to watch out for on free forums are the database export options. There will come a day when you want to host your forum yourself. Most free hosts do not allow you to export your database, meaning you are forever locked into their service.
I was on forumotion for years when I first got started. I found out the hard way that you can't leave when you outgrow the service. There are ways to get off anyway, but they are complicated and extremely technical. And many forum hosts don't even have an option at all.
Really, if you're looking for a free host to get started, the *only* thing that truly matters in the end is if they allow database exports (whether paid or otherwise). Some of the smaller ones allow them, but some of the larger ones tend to restrict them. Besides that, it doesn't really matter which one you use, besides the features that they come with of course. Pick the one that looks the nicest to you and roll with it.
Free forum hosts aren't the best, but they are perfectly fine for getting started. What matters is your ability to run a community. All the features and styles are secondary, but make absolutely certain you can leave if you want to. Otherwise you'll be locked in even after you outgrow their service. Just do your research. There are a few (normally smaller ones) that allow database exports, and those are the ones you want to use.