Well, they're not toys, but they're not quite serious synths/keyboards either. I'm not quite sure who Yamaha are aiming at. They're very well built apparently, but possibly too well built and too well specified for the casual and hobbyist market at an expensive price compared to similar stuff from Roland and others - but they don't have the necessary for serious work either. And why 128 note polyphony for a monotimbral instrument? Hands up anybody who needs 128 notes at once
Yamaha are obviously trying to cater for the live market as well by giving them DIN MIDI capability as well as USB MIDI so you can play them from another MIDI keyboard, but with their small size and fiddly MIDI adaptors, I don't see many takers. Too much hassle, and nothing you can't do with existing keyboards.
At the quoted price you're not going to buy them casually and sling them in your backpack for school or college, and "collecting the set" will be prohibitively expensive. $3000 plus for four? Yikes. You could get a decent workstation for that money. But they seem to sound pretty good, so who knows? That might trump everything.