Two items on wounded units in FF:
First, I noticed there is a small difference with FFH (and vanilla Civ, I think) when you select the heal icon in FF. In FFH that unit will heal/sleep until it is fully healed. However, in FF I find that despite not being fully healed the unit will keep waking even though damaged. One of the first times this happened I wasn't paying attention, thought the unit was healed, and sent it to its death in a battle it would have easily won if healed.
Not really a big deal unless you are swarmed in your only city by barbs or enemy units and your units keep waking rather than fully healing. Maybe it is the same mechanism that causes units set to sentry to wake as it happens when other units are around?
The second thing I think is in FFH too. That is the effect of poison on units. Personally, I think it is overdone, especially with HN units. For example, I had my HN Giant Spider defend and kill a Scorpion Goblin. My attack odds were 99.9% had I decided to go after the Goblin. How many turns did it take the Spider to fully heal? How about 36 turns? I just don't get why poison affects HN units so much more than regular units. It is frustrating enough to get a damaged HN unit in non-friendly territory and see how long it takes to heal.
Even further, one time I was lucky to get my HN Griffon mutated from a Shipwreck. This included Poison Resistance, but that didn't seem to work at all when it was poisoned by those Scorpion guys. It was still many, many turns to fully heal.
Finally, don't ever get a poisoned, withered unit or, I have found, it will never heal. As far as I can see you cannot get rid of the wither promotion and the poison combination will keep the unit's strength at 0.