I totally agree.He’s not wrong though. I mean, not really enough info in the OP to make much of a definitive statement (not blaming OP, who seems new to this and can’t be expected to really know). But:
-shop can’t put a bike together “without a part breaking (what part? Even cheap parts don’t break during assembly unless already broken or the wrench screws up).
-tensioner broke (ok, sucks, but chain tensioners are 10-20 dollar parts that take 3 minutes to install assuming it actually broke and wasn’t just misadjusted or something)
-chain thunking during initial test ride (this could be one of a hundred different things, from something catastrophic failing to a misadjusted derailleur)
-brakes losing efficiency (are the discs bedded in? Disc brakes take a bit to bed in properly. Or could be cable housing settling into their ferrules which will loosen up the cable a bit and make the lever feel less effective).
Not to dogpile on the OP or anything, but honestly some of this sounds like a shop that half assed the assembly and is blaming the cheap bike for it.