Do you have thoughts on RPG mechanics that make units weaker as they lose health (e.g. injury, vulnerability, loss of morale), how scale (sheer number of units controlled by player, enemy) affects their usefulness/viability, and/or whether, in your experience, they're actually fun?
That's a suuuuuper broad question! I'll start with what I think you're getting at in the first part of it. I know of a few games that make units deal less damage as they lose health (King's Bounty, Heroes of Might and Magic, Advance Wars, and The Banner Saga all come to mind). Characters losing fighting power along with their health tends to produce a positive feedback loop wherein they become less able to fight back, thereby making their eradication more and more likely.
In games where characters are replaceable, like King's Bounty, HoMM, and Advance Wars (in Advance Wars, they're replaceable during the actual battle!), you're really dealing with
groups of soldiers in each unit, and the unit's "health" is effectively just the number of soldiers left in the squad. Your ability to maintain a flow of replacement troops is part of these games' loops; it's just part of the macro-level strategy, so I find the mechanic tolerable in these circumstances.
I really quite dislike it in The Banner Saga, though, as TBS is really just a tactics game: your units are all unique and more-or-less irreplaceable. Since you can't easily recruit replacements when you lose soldiers in The Banner Saga, the positive feedback loop that ensues from units weakening as they become more vulnerable has
extremely severe consequences without any macro-level upshot to offset it, and I tend not to find it terribly enjoyable.
Morale is an interesting one. I've generally encountered it in two forms: (1) in battle, as a way to take control away from the player right when they need it most (i.e. when they're losing); and (2) outside of battle, as a way to punish the player for failing to pay/feed their troops. The first use I find irritating, as it's another example of a positive feedback loop that hurts the gameplay experience. The second use I tend to see more in strategy games, and it can be implemented well in that context. One new use of morale I've seen is in HoMM7, where it's used as a sort of bonus to grant your units extra turns if you're doing well--it's overpowered in that implementation, but I like the general idea of morale as a bonus to how well your troops perform. It's positive feedback-y, sure, but it's positive feedback-y in the player's favor, which makes it fun. (In my experience, things which are patently unfair when granted to the computer--like critical hits--are enjoyable when granted exclusively to the player).
Scale...that's a really broad topic, and it's hard to make any sweeping statements about it. I've seen both large-scale and small-scale done well, and I've seen both done poorly. In general, I'll just say that the fewer units you have under your control, the greater a variety of tactical options you need to have with each of them to keep things interesting. You can get away with less-versatile units once you get up into the 7+ unit range, I think. In general, I think the usefulness and viability of a given unit has more to do with the game's underlying mechanics than it does with scale on its own.
