MUI is not only about good looking windows but it's a framework to ease GUI development.
...
AFAIR, you needed a license key to be able to use MUI 3.8 for GUI development.
Please tell me, where can new developers obtain a license key for MUI nowadays?
Stefan Stunz does not reply - even not if you bribe him with money.
And the "MUI for Amiga-OS"-developer team does not seem to be willing/allowed/able to generate and sell license keys for their own piece of work.
So, GUI development with MUI 4.0 is only meant for old developers, that already have a license key, right?