No reviews, and has only been out since January, so it's unlikely you'll find many, or any, members who have purchased one yet. Also, since it's so late in the game for the B450 chipset, with newer chipset boards on the near horizon, it's not terribly likely you WILL see that either.
Anandtech breaks down the boards features, although it's not a review in any stretch of the imagination, here, and there is some good commentary afterwards.
https://www.anandtech.com/show/13902/asrock-unveils-b450-and-b450m-steel-legend
Seems it has mediocre audio codec, only a 6 power phase VRM configuration, so it's not going to be a very good overclocking board, DOES have four DIMM slots, which is unusual for an mATX board these days although some do have them but a lot have only two. Has two M.2 slots which is nice on an mATX board especially if you plan to use a small enclosure and want to eliminate drives taking up space, however one of them ONLY supports SATA M.2 devices unlike what has become common now with dual PCI M.2 support on both slots.
Overall, probably a decent budget option, if you don't plan to overclock and don't care about higher end audio codec onboard. ASRock makes quality boards, but you can probably get a much better board for 20 or 30 bucks more.