Been watching through footage of the Nintendo Live event.
https://www.youtube.com/watch?v=-QyQWFVRDQE
Some observations for meter final smashes:
-First, the things we already know: meter final smashes are weaker, and only one can be performed at a time.
-You get meter for lots of different actions, including successfully landing hits, getting hit, shielding attacks. So it's not purely a comeback mechanic, or a pure reward for aggression either.
-Multiple characters can have meter final smashes charged up at the same time (so it's not like the Smash Ball, where there's only one in play at any given time).
-If you get KO'd while your meter is only partly filled, it adds to your meter. However, if your meter is filled up and you get KO'd, you actually lose a bit of meter. How much meter is lost when you're KO'd seems to vary for some reason. I've seen characters lose anywhere from like 15% meter to 40% meter.
So there's a lot to the mechanic, and also more to discover. Viability for tournament play might just come down to how powerful the strongest of the meter final smashes are. If they're not overwhelming, it could actually work.