1. Keycap compatibility. Especially those with weird size spacebars or unique sized keys like Esc (Choc Mini and Pure Pro). IMPOSSIBLE to replace all the caps on those. The Keycool 84 is better, standard size spacebar, Esc, etc, but the edit keys are weird profiles, so you have to either settle for wrong legends and right profile or right legends and wrong profile. Or use a uniform profile set like SA all row 3 or DSA.
2. Looks. I think they look messy compared to 60% and cramped compared to TKL.
3. Awkward position / size of important keys like right Shift, Fn, Alt Gr (for those of us who use other languages), etc.
So I use a 60% despite the slight awkwardness of not having arrow keys on the default layer. The Leopold FC660 layout is WAY better than 75% and solves this problem, but it has a sucky Fn layer, whereas the Fn layer of my KBT Pure 60% is great.
P.S. - that MX-Mini is the same layout as the Keycool 84, just with a nicer / different case style. Still NOT standard mods, they're 1x size on the right and there are only 3 instead of 4....