Okay, that won't elicit a short answer. But I'll share what I know and have heard.
Markins generally is thought of as not playing well with other gear. For me that's critical because no one manufacturer has everything I need. It's amazing how important a millimeter or the cant of a dovetail is to getting pieces to mate easily, and Arca-Swiss did not produce universal standards. I've heard folks grouse about several Markins ball heads. The only Markins head I've heard recommended is the Q20, but you might still have trouble making that work smoothly with other gear. The ball head is the most important component of a support system. There are better places (eg: lens and body plates, clamps, flash brackets, macro rails...) where one can save money. I recommend investing in a top ball head, then saving elsewhere.
I've owned a Q10 for several years and researched it heavily beforehand, and this is the first I've heard of those incompatibility criticisms. I use a mix of Markins, RRS, and Kirk camera and lens plates and all of them mount easily and tightly on the Q10 with the screw clamp. Lever clamp bases are always tricky to mix/match plates on, so perhaps that's what the fit comments you report are based on. However, Markins answered that by making their lever clamp tension adjustable, unlike RRS which I have heard is very picky about fit with other AS plates.
The *only* consistent negative people report is difficulty really locking down the pan base, particularly people who tote their gear mounted over the shoulder. I haven't had major problems with that issue, but I it does take a pretty good twist to get the pan base tight so I can see how that happens.
That aside, I think the Markins Q10 is a top-notch ballhead that operates very smoothly with no sag except under extreme conditions. It's much easier to find and set the sweet spot than any ballhead I've ever tried. I only experience sag once, and that was with a 180mm macro plus 1.4x extender way off axis on a set of RRS macro rails and with a 430ex flash mounted way out above the lens. It held great, despite all that torque, I just had to deal with minor sag when tightening. This is much, much better performance than the Kirk head I upgraded from.
No experience with the BH55, but there is a major increase in weight and a bit more $$ with that head, so I've stuck with my Q10 even when upgrading the 3-series RRS legs. I'm a big RRS fan, though, so I may end up there some day.