There are a few sites where this happens more often than others, but it's never happened here at MobileRead for me. But if you use Firefox, try the addon
Lazarus. Even if your browser crashes, you can recover all your text you typed but didn't submit through a right click menu with Lazarus. It's great, I've been using it for years.
I don't think there's anything wrong with any forum software. Some sites just log you out more often for safety reasons.