Great bang for the buck is, IMO, highly subjective. Granted, for a budget entry it's probably fine. It also seems that it doesn't specifically have significant throttling issues. However, reviews seem to suggest that there ARE other concerns with the B450 Mortar.
The power consumption numbers were a little high; at stock speed the idle draw was 97W, while the Gigabyte board came in at a little over half this amount and was noticeably less power hungry when overclocked too.
Really, there are few professional reviews of that board that are reputable. A couple, but mostly there are only a few and most of them are quick comparison reviews of the paper specs for ten or more boards from that chipset like the one on Anandtech. And usually, when there are not at least two or three GOOD reputable reviews, it's because reviewers don't feel like it is worthy enough to warranty purchasing one for review AND/OR the manufacturer doesn't particularly want that model reviewed because it knows it has flaws that will be likely to come into the light if it gets looked at too closely. In other words, it usually means "not worth reviewing".
I'm not saying that's the case, in THIS case, with THIS board, but in general, that holds true.
That being said, it does seem like the VRM configuration on that board holds up well in the review sample on Bit-tech, but that is the only review I see that is from a fairly reputable reviewer and in any depth at all. I don't particularly trust singular reviews. We've seen problems with that before like the EVGA B3 series power supplies that were touted as being a good value by a number of sites, and yet when Aris got around to testing them legitimately, all but one model in the series failed testing horrendously. So, if there are not multiple reviews I have a hard time recommending them, but again, I don't have a lot of concrete evidence to mount against that board either aside from MSI's reputation and history when it comes to their budget or lower mid tiered motherboard product stack. They got better for a while, but I have seen a trend back the other way for a while now.
Maybe it's just me. Maybe it's not.