Our backup/rpc node can be used on (though from time to time we switch to the one provided by engine's team when we are doing upgrades/taking backups). The backend for NFTMart is fully powered by our rpc node. Some witness nodes block off their RPC port so you can't get data out of them. Its like hive nodes, where fullnodes can be used for most reasons, witness nodes usually can't.

Ahhh, so unlike PeakD for example, it's more dependent on which website you use and they can't be changed on

As of now it can't. But if sites add an option to switch nodes then they will be able to. Since more nodes is a new thing, it'll be a while before they get implemented. A lot of the tools I've made for engine require on our backup/rpc node.