HP thresholds vs per-round actions

I was looking at the Zombie Clot adventure (really, encounter), and it has a feature I was pondering: at initiative 20, it spawns minions. This has a snowballing effect, because you get a number of waves of minions equal to the number of rounds it takes to beat the boss. This means that if the fight was already hard, minions make it much harder, but if the fight was easy, minions barely affect the difficulty.

There's an alternative method, commonly used in video games: effects (such as minion spawning) occur at hit point thresholds, rather than based on timers. This generally makes difficulty more consistent and makes "focus fire the boss" less obviously optimal tactics. This can also be used for other abilities (for example, rather than giving a dragon's breath recharge 5-6, give it recharge after a short rest or when first bloodied). On the minus side, for a lot of creatures this is quite gamist.

So, how do other people feel about this kind of threshold mechanic?

Well, focus fire on the boss is an effective strategy for a number of reasons. Summoning minions at HP thresholds won't deter anyone from that. I don't think anyone has the mentality of "Oh man, damaging the boss summons minions, so I guess I just attack something else" The boss is a dangerous creature usually, and defeating it is a priority.

It is an interesting idea though. I might try that out. I do think that limiting the creature's signature ability to thresholds is a double edged sword. It makes it more cinematic, with heavy blows being retaliated against, but it can also limit the monster by a lot. Giving a dragon 1-2 uses of it's breath per use can destroy it in some encounters, like a skirmishing dragon running hit and runs against defenses or in the wilderness. I think maybe keeping the 5-6 but also adding the bloodied thing would be a good indicator to the players that they're making progress, but I think that if you really wanted to emphasize that, you should add more than just an automatic breath recharge. Perhaps also an increase to speed, armor class, and/or a new ability would emphasize that kind of threshold combat a little more.