I'm a GURPS guy and not terribly fond of the d20 in general. However, this game seems quite interesting. My group is going to, after our current game goes on its break, give this a try. It looks interesting. It has a lot of similarity to D&D 5e, but, really, in a good way. The short description is 2d20, roll low, success-counting. Each die is read separately and can generate its own successes. Successes are counted compared to a required number of Successes. Extra Successes build up Action Points which can be used to do other things. Damage is handled with specialized d6’s, but, they’re easy enough to emulate with normal d6’s. It’s like every other 2d20 system out there. Just a bit more crunch added. But, honestly, not enough crunch. But, take that with a salt-mine, I am a GURPS guy, after all.
Mechanically, the only thing I really have a problem with is the horrible "area-based" combat. It barely worked for FASERIP Marvel back in the 80's, and it doesn't work for this game. In one scene, you can shoot across a city block, in the next, you can't shoot across a room. Areas are bad. It doesn't fit for Fallout--they need to spend some time and get an actual movement/range system. Me, I'm going to be cribbing from D&D. I thought about cribbing from GURPS, but other than using hexes, D&D is a better fit.
Beyond that, the book has a bunch of errors that have been reported, but haven't been corrected yet. I did reach out to the publisher and they told me that since the game hasn't gone into full release yet, they haven't updated anything. I deeply appreciate that they took the time to answer the question of internet guy. But, the decision to not update is a bad look for the developer. If they're asking for money, this shouldn't be selling the play-test documents. At least, not without saying “hey, these are the playtest documents, if you go here you can give us feedback.” At the very least, they should have an official errata document bundled with this indicating that they are aware of the error, and will be correcting it in the future. One of the best “source books” I can suggest is getting the Fallout 4 GOTY Strategy Guide and using their cost/weight information for anything that seems hinky (because it’s probably typo’d or two columns in a table have been swapped! I’m looking at you Powered Armor table!).
All in all, the game looks interesting. Experienced GMs will have no problem fixing the bugs in this game. Much like the CRPG Fallout 4, this game will be made much better with community-provided Mods.
[4 of 5 Stars!]