Given the variables in what is actually in the ceiling behind it, is it better to have a small enclosed speaker that you just hang from the ceiling rather than an in ceiling speaker?
No, both are roughly equal in trade offs from 'hifi'.
What is behind the wall doesn't matter in 99% of cases. Wires, plumbing,most types of insulation, wasps nests - none of this matters. The inwall speaker just need enough breathing room, ie, the enclosure size(wall/ceiling cavity) is correct for the woofers T/S parameters(should be large in the case of most in walls). So if the cavity is small or say filled nearly full with some sort of closed cell foam it would not be a great match in the bass and mid-bass ranges.
Speakers mounted near the wall suffer from SBIR(speaker boundary interference reflections) in the midrange and comb filtering in the midrange. This is possibly the worst spot in the frequency range to have these issues. While such a small speaker may measure well on the klipple, put it in that location and it will not.
I think the choice comes down to what is easiest for you, looks the best and costs the right amount. The height speakers are simply going to have issues, but they are not 'hifi' speakers they are height channels.