Some of the games are immense - full on theme parks, weird physics games, movie tie-ins, "total wipeout" physical games, all sorts. There was a really accurate Natural History museum at one point. It's so incredibly diverse, that it's hard to get a real handle on it unless you play it.
It seems to have got real traction with some of the kids in terms of the effort they put into programming their games, and hopefully it'll be a gateway to programming for some of them.
I think kids spend too much time playing games, but Roblox is pretty good, and they even make an effort to censor the chat - which can never be 100% but it's better than a lot of places.
FTR I've played minecraft with them a lot too, but the issue is that ends up being a fighting game ones the boys' hormones kick in (sadly!)
While Minecraft has sort-of become a platform too, with people doing custom servers etc., Roblox is explicitly built to facilitate being a middleman skimming percentages of people buying and selling digital assets and game passes.
The marketplace added to the Bedrock edition of Minecraft is a baby step in that direction, but nowhere near as flexible.
If I had to compare to something, I'd say it's like Unity, except with their own app store from which you can instantly launch games/experiences, and with cross-platform multiplayer taken for granted.
What I love most about Roblox is how the community is leaving behind a lot of the old game tropes and coming up with completely new things, as a lot of the game makers are kids. Turns out most games don't have to involve shooting anything.