The exploitiveness of an individual Roblox game really depends on that game's developer. They get to decide how to extract money from their players.
I've seen a lot of different sales techniques used. Most common are pay to get an overpowered weapon or ability or to skip levels.
The ones that annoy me the most are the artificial scarcity/limited time only sales techniques. "Adopt Me" is one of the most popular games. Their main product is digital pets that cost about 12USD, or upgrades for your pets. They have rotation of pets that are only available for a limited time. My kids are really suspectable to that technique.
There are also some games that are obnoxiously full of unavoidable little floor buttons that when stepped on will trigger the confirm purchase dialog. The dialog is easy for kids to learn to just-cancel, but the developers who use this technique seem to have real contempt for their players.
That said, there are tons of fun games that you can play without spending any money or being exposed to obtrusive ads.
My favorite game is called Secret Staycation. It's really well made, and there are zero in game purchases. I only saw one ad in the whole game and it was for a physical plush toy of one of the game characters.
Also, shame on Mark Rober for his buggy abomination of a Roblox game. That felt gross and exploitive.