Fly is the single biggest spell disappointment. It highlights that the engine is just a polished DOS engine - and really they should not have included it or at least called it something else (they already have a jump/ability spell which is very similar). I also disagree that it's current form is more than a technical issue - all the "issues" that were raised can be countered with slightly more careful designs and systems. If you *know* people can fly, then you change up encounters - and you use fly against the PC and their party too. Also, you need to *concentrate *to fly - if someone damages you with a ranged attack, you can lose that fall to your death. Tradeoffs. Also...the game that is much maligned - Solasta - dose implement 3d fly, which while slightly wonky at times, is amazingly fun and shows what could have been.
