Hello Angelo,
I'm glad that YEM is fulfilling your needs. But the thing is, the software is called "expansion manager" and in my eyes it doesn't really deserve that naming -considering, it's the only software Yamaha provides for keyboards.
Let me give an example...
You load some pack into YEM and that pack contains manyl voices and styles. Now let's say, you only like two styles, which you wish to install on your keyboard. And now the question: how do you know which voices (in pack) you need to add, so your two styles will sound properly? The answer is: You just don't know -because YEM doesn't provide that info. Means, if you don't have some other software by hand, you're forced to install all voices inside pack, even if you only use one style. And the problem becomes unsolvable in case of protected (cpf) packs. That is just one example of what YEM can't do (and it's such a basic thing).
So yes, there are objective reasons to complain about YEM.
Bogdan