When I've built things with them I've mostly considered the quirks defects.
Kind of like how httpds will have quirks but those aren't really a good thing and they're kind of plug and play.
What kind of quirks have you seen that the next model wasn't better at?
A simple example would be when models get better at following instructions, the frantic and somewhat insane-sounding exhortations required to get the crappier model to do what you want can cause the stronger model to be a bit too literal and inflexible.
I don't mean to be too pointed here, but it doesn't sound like you have built anything at scale with LLMs. They are absolutely not plug n play from a behavior perspective. Yes, there is API compatibility (text in, text out) but that is not what matters.
Even frontier SOTA models have their own quirks and specialties.