Are disc brakes weak? And what is "bedding in"?
Bedding-in is a common process with new disc brake pads. Disc brakes were developed much earlier for motorcycles and cars, from which the technology and the terminology has been transferred to MTBs: see https://ebcbrakes.com/articles/motorcycle-bed-in/ and http://www.centricparts.com/files/White%20Paper%20Revisions%204-2012/Centric%20White%20Paper%20C3-2012-Bed-In%20Stock.pdf

Bedding-in is a process in which both the pad surface microstructure but also the surface composition of the rotor is affected. During the run-in a very thin layer of material is transferred from the pads to the rotor. This surface deposit increases the brake power significantly. 

However, even when the brake pads are not yet bedded-in, a disc brake should have more stopping power than a rim brake. So I wonder whether the surface of the rotor or possibly the pads have inadvertently been poisened by a lubricant. Ask the seller to clean the rotor with alcohol.