Don't think of the monophonic aspect as much of a limitation. If you really want poly stuff, you can layer multiple recordings.
But in exchange for monophony, you get a powerful analog synth engine; it feels natural, the range of basic controls that all contribute to the final more complex output. It really feels like an instrument, with the knob-per-function controls mostly directly interfacing to the sound generating circuitry inside. There's no presets or digital control of the parameters of the synth, like there may be on others, but that's not the point. You learn your way around this instrument as you would any other. With practice and experience, you can dial in a sound you're imagining in your head not too much slower than it would take to load a preset.
As for quality control, you'll see probably most of the posts on this forum noting flaws or defects. But really, who is going to post whenever their minibrute is working perfectly? Of course you're going to see many problems on the forums, that's what they're for, getting community help on issues. But that doesn't mean the majority of minibrutes have these issues. (a lot of the problems are just due to loose cables inside the machine, a simple repair)
I like the build quality of the MB. It's not Mackie mixer rock solid, but it's not flimsy. I'm careful with it, as one should be with any gear, but I'm not worried about it breaking on me.