I have nothing for you on the RDB, but, I used to shoot with a guy that had a RFB. Shot with him for over a decade and he had only one self made issue, imo. He would screw with the adjustable gas port to the point of making the weapon fail. He also shot all the left over junk imported 7.62x51 ammo he could dig out of his old stock, and then mix them in the mags. It always made me giggle when he wondered why it wouldn't group to his satisfaction.
I thought the weapon was cutting edge production, as I have thought of Kel Tec ever since I bought their SU16C around twenty years ago. From my experience with that piston AR imitator, I believe Kel Tec tries very hard to produce a no nonsense, cutting edge, minimalist style product for the most competitive price possible.
The SU16C never failed me, and never failed to be accurate for many, many thousands of rounds. Another one of those rifles I wish I had never traded it off. I have owned a few Kel Tec pistols over the years with the same feeling of satisfaction for a moderate priced product.
I wouldn't think twice about buying any Kel Tec product, and their customer service was excellent, years ago, I doubt it has declined. Hope this helps.