I'm not arguing, right, but what differences would you point to as having like, balance/functionality implications? I'm struggling to think of any but it doesn't mean they don't exist!
The main difference I can see is that a solo which just like, has like 18 HP (instead of 6), say, would probably be kind of boring to take down, because that's 6 to 18 hits (and probably at least 12 unless the PCs are rolling amazing damage), and it'll have like, what, probably 8-15 spotlights from the PCs failing rolls/rolling with Fear in that time (I'm assuming they'd be working to get Advantage, using Experiences etc. to reduce the outright fail rate in such a fight), and if it's basically only got a couple of things it does, you might have to work pretty hard to make that interesting.
Whereas if you have a "serial" solo which has three stages (each technically a solo with 6 HP) with different abilities/characteristics each, I think that's probably going to be more inherently interesting and a little less work to keep entertaining. I'd probably include verbiage that cleared CC "for free" on a stage change.
Just thinking about the sheer number of spotlights you could potentially have on the same monster I'd definitely save 3x HP/triple stage solos for like, really climatic boss fights, probably once every few sessions.