Here are the potential issues you will face:

No DRM Content. (This includes no DVD player, No DRM content online, no Music Videos from Apple Music, no Movies from the TV app, no photos curation, and no maps {maps do work on macOS Big Sur})

Glitchy apps and animations. (Especially in the Menu Bar. There will be several glitches when using apps with no Metal.)

Apps Unavailable: (Some apps are completely unavailable without Metal. {Such as Garage Band, iMovie, Photo Editors, etc.})