MSX1 vs C64

That sounds correct — it's exactly the same thing as, say, the floor in Street Fighter II, but without any vertical motion.

I'm not that impressed by demo effects though. If we're counting demo effects as fair representations then here's what amounts to that for the AY — it's the Atari ST introduction screen for Xenon 2, using no doubt a substantial proportion of the available memory for sampled sound. At about the minute mark the user presses 'space' to load the main game, and the space and processing time for audio is reduced to realistic in-game limits, so it transitions awkwardly to standard three-channel stuff.

That's the difference between a demo effect and a real deployment. The C64 demo scene's impressive output doesn't necessarily say to me that it's an impressive machine, just that it's a machine with an impressive demo scene.

C64 did have cool game loaders, and music could play whilst loading (and saving?) data. I always was jealous of that ;-) (Oh, we've got this (courtesy of @manuelbi) ;))

Something else that's notable: it seemed common on C64 to only have music on e.g. the title screen, and in-game sfx only. Or music in-game as well, but then no sfx. Rather lame of course. (But conventional?)
An exception (but perhaps there are many, I'm not sure) is Monty on the Run - which has in-game music, and sfx plays when picking up a coin (but it takes 2 (I believe) channels of the BGM).

Simultaneous BGM & SFX easier to pull of properly on the AY-3-8910? Or also a 'cultural' / difference in convention here?

Btw: SID ;)

Thanks TomH.
Btw, in BIG favor of the AY-3-8910: I like this* one a lot. Perhaps the best (scene) PSG thing I've heard?

* Xyz - Well Beefington

My argument is that they're primarily sleight-of-hand, and geared towards doing things that are unrealistic in any other context. Most of them boil down to being a sufficiently-compact way to replay their selected precomputed effect.

I guess my argument is slightly philosophical: I'm taking it as implicit that one of the defining characteristics of using a home computer is that it is an interactive experience, and the quality of a computer is the quality of that experience.

In my view, because demos are fixed audio-visual artworks, they're merely a sideshow. If you were in the 1980s and wanted to buy the thing that was best at replaying a fixed sequence of images and sounds then you wouldn't buy a Commodore 64 or an MSX, you'd buy a VCR.

Games and applications are the real items of interest for me. I'd much rather play a game I don't even really like than watch a demo.

In the end IMO it all depends on the composition. On both SID and AY3 and OPLL I’ve heard astounding beauty and utter garbage . On SCC, of course all music is beautiful, but maybe that’s because Konami made most of it .