The MBUS loses precision due to the small aperture hole being slightly larger. Standard A2 small apertures range from .060"-.065" (manufacturer dependent). MBUS small apertures are .070" (so are Troys, for that matter). When talking about precision, hundredths and thousandths can make a difference.
The large MBUS aperture is .185", whereas most other large apertures are in the .190"-.220". This relates to targeting speed, but it's more shooter preference than anything, however.
Then, the MBUS Gen1 apertures were not on the same plane, but the Gen2 solved that issue and put them on the same plane.
If you do a search online, you will find reported issues of aperture sights on MBUS rear sights being off-center, but with the incidents I've read about, Magpul has always done their part and helped the customer.
I also disliked how the front sights were too "open" with the design between the side posts, but the Gen2s have resolved much of this issue.
Here's the thing- this is really nit-picky stuff. I realize that. But when someone says "open sights" as opposed to "BUIS or back-ups", I think of primary sights, not BUIS. I would make the argument that there are better options out there. If one wants iron sights for primary use, they should be using fixed sights, not folding. Daniel Defense, Troy, YHM and other companies make fixed iron sights that would do well for primary use. In tactical use, they work very well. If you want more precision, it's very hard to beat an A2 setup.
Such has been my experience. YMMV.