Frankly I don't care why it sucks, because all that matters is that it sucks. I would literally take a pay cut if it meant I could use a Macbook Pro instead.
That is a lucid, intelligent, well-thought out argument for not buying an X1 Carbon with Windows 10.
Just kidding of course. "Sucks" is not a description that has value in any meaningful way. I use an X1 Yoga constantly with Win10 and have no drama. So while there are varying experiences with Win10 and Lenovo, "sucks" hardly explains any of them.