They are fine as long as they are supported properly - hence having the foam or other padding under them. If the stand isn't FLAT then you end up trying to bend the glass. Then BAD things happen.
The stand is steel with 3/4" ply on top so it supports the stand fine. If they do fail it's usually because the bracing at the top comes off or gets broken. That lets the front glass bow and crack.
I imagine that even support would be more critical with the all glass tanks than with a framed ones of course.
Next project is to build some nice wooden doors for the cabinet, make it look a bit better