I am also not loyal to a brand, but buy what fits my layouts theme and era.
Of my Factory sound equiped, diesels:
ScaleTrains - 1 - Very nicely done. Would have loved more, but they are pricey.
Walthers Mainline - 1 - Less detail, but very smooth drivetrain. Price was good. (Soon to become 2.)
Bachmann - A handfull of NS Heritage units, all found on a closeout sale, All but one at under $100 each, brand new, with factory sound. I have no running issues with them. One has a decoder defect on the ditch light circuit, but still operates smoothly. My one Bachmann Steamer also has sound, and is a very smooth runner. (NKP 765. Had to get her.)
Intermountain - 2 - Both well made, but a light pipe issue on the one. Solid runners though.
Bowser - 4 - Some of my absolute favorites. (Soon to become 5.)
AthearnGenesis - 2 - One great, one had the old bulb lighting, but runs well. Those bulbs die way too quickly though, and are a royal pain to replace.
Atlas - 2 - One QSI, one ESU. ESU one certainly runs much better.
FoxValley - 1 - Aside from a bad DOA ESU decoder, runs great.
Athearn RTR - 1 - Decently done. Less fine details, but the recently designed with LED’s are smooth.
BLI - 2 - One DOA, but that was a issue of a seller online sending a known defective in place of the as described NIB. Other is smooth runner. (Was refunded and told to use for parts once fraud charges were brought up as a possible recourse when he refused return.)
I have more non-sound, from Bachmann, Atlas, Athearn, Stewart (1), Proto, and Intermountain, and some will be converted eventually.
But overall, I’ve not noticed any mechanical issues in diesels in the last 10-15 years.
You might find issues with older Bachmann, Proto, etc… But even guys like Athearn & Atlas have had clunkers in the past. And all manufacturers could have a one-off bad one.
I have never had any locos from Rapido nor MTH.
Steam -