Outside of the (unfortunately) inevitable disc read error that plagues them and an issue I had once where my disc tray got stuck due to a tab coming loose, I haven’t had any issues with my fat model so far. They’re not as bad as, say, the old fat PS3s for example - those are pretty much ticking time bombs that probably aren’t worth the upkeep costs.They're generally poorly built and tend to have random hardware faults in my experience. Slims are just better. The Slims have the obvious problem of no IDE connection (although some still have it in the board), but compared to USB Ethernet is serviceable.
Slims are still better though for the most part though, I agree; the massive size reduction and better build quality is really nice to have. The aforementioned lack of a concrete solution for hard drive use is a huge shame, though.