So, if they have a line "You're not able to post links to other services in your profile" in the agreement, then everything would be ok?

In principle: yes. That is, as long as the agreement is not in violation of other rules/laws. Considering the role a service has in society and its market position, such a rule could e.g. violate anti trust law. But that's a different matter, and up to a court to decide.

It's crucially important to take managing and lifetime of such storages into account. I doubt it can fight with SSD or HDD.

I tried it multiple times and it really depends on the game. If there's an opportunity to move fast (faster than in real life) then I definitely feel sickness. The same for friends who tried it too.