They usually destroy themselves in acutal operation, often after a short, power interruption, or reversal. They will lock up and only run in one direction or just sit and smoke. The relay will chatter, or the 7800 series regulator will fry.
I bought 10 or so years ago for really cheap, and have blown up about 4 of them. I still have about 5 or 6 that I use with the knowledge they will fail. You can ignore my experience at your peril.
They also do not meet NMRA standards for max track voltage.
Greg