M&M allows for the widest range of economy booking classes to be upgraded.
As you already saw, generally Y, B (the 2 highest Y booking classes) and G, E (the 2 highest premium economy booking classes) are usually eligible when using miles from other programs than M&M (with a few exceptions like UA, which you also found).
It usually really isn't worth it using miles to upgrade, especially now when cash upgrades are offered at relatively low prices.
Of course that would certainly change if the travel ban between Europe and the US ends.