SOrt of depends on how good the bracing is, but fiberglass' biggest issue is flexing and the development of spider cracks, which can lead to catastrophic failure, and starts with just looking lousy. While easy and cheap, the stuff to build an actual tiled shower pan isn't all that expensive, either, assuming you're willing to do it yourself. I think I'd try to fill those voids with mortar. You didn't say anything about level, only flat. It's critical that the pan be level, and mortar can allow you to do that, should it be needed. You don't really need to protect the floor, but you do want some moisture barrier underneath - it will help to hold the moisture into the mortar mix so that it can cure with max strength. Otherwise, the plywood floor can wick too much moisture out, which leads to weak mortar. ALso sort of depends on how deep those recesses are and if it does need to be leveled.