Whatever you are using from a third-party source is dangerous and goes against EULA and no manufacturer support if your device gets problem after using it. Youtube ++ is a modded version of youtube and it's a third-party app. Literally, you are using an illegal app but Apple has not ever got legal action to suppress such apps. So, you do not have restrictions to use but no official support either. It's up to you. As far as I know, there is no harm reported yet related to Youtube ++