I generally don't have any issues with Denon AVR's. I've replaced one in the last year due to lighting strike. I've only sold 3 of the Marantz receivers in the last 6 months and had one issue out of the three. It had an issue processing 4k, called Marantz and they told me to use the closest HDMI inputs to the output port for 4k content. I switched the input and it did improve but was still a downgrade from the client's Integra. Marantz always sounded better to me then Denon but they never seem to get the video right.

Every company has issues... just some really stick with the brand. Onkyo / Integra had a disaster that, even though the product has changed since, they still haven't recovered from.
Denon have had some noticaeble sales success in the US of A, I do put quite a bit of it down to the grand standing on the likes of AVS Forum from their dealers like JD Smoothie etc, these sites influence more than many people believe.
For me...
The more channels you jam into a Class A/B receiver, the more issues you'll suffer from. Sure putting a fan inside will keep it cool, but the fans will grow noisy over time and in turn the amps are almost always dust magnets that will then cover the internals and cause more heat / shutdown issues.
But if an amps is failing weeks / months out of the box, yeah, talk to the techs and not the reps - they can't help but be honest.

More than a few times I've had a new Denon out of the box and had to factory reset it to get it working properly.

If they still had in-person training, you would have known that and if you have attended their training in the past, you should have remembered them telling us that the first two things we do after opening the box:

1) Hard reset
2) Firmware upgrade

Regardless of brand, I always update as an early installation step. In a few cases where I track forums daily, I may hold off on a recent update if there is a known issue.

OUCH!! After having taking a Denon AVR to local warranty repair three times to fix the same channel we were done. Repair shop defended Denon and said it was wire or speaker. We replaced the speaker, couldn't replace the wire but really didn't think it was our wiring.
After two repairs we put in a Pioneer unit and never had a call back!!! Gave the Denon to my son (after the third repair), first time he tried it, it smoked and he tossed it. No more Denon after that!!!

That's my experience, too.
We recently installed an RX-A780 and had difficulty getting Zone 2 to come on. We ended up talking to Yamaha, who told us we had to select 7.1 + Zone for zone 2 to work. I swear we tried that during the first DAY WE LOST trying to check everything on the unit, but Zone 2 just popped on when we selected that.

I put that down to a manual that has instructions for Zone 2 spread out, piecemeal, over a half dozen pages. They would do well to leave all those notes everywhere but also put them ALL together in one section!

Power amp assign can confuse.
Especially if people think they've setup Zone 2 via the MusicCast app... but can't get any sound to come out of the Zone 2 speakers!
Having said that, I prefer to use WXA50's stacked for MusicCast installs as network streaming autonomy is far more important IMHO than a single box Zone 1 / 2 install IMHO.

Sounds like that service center hired the technician that I let go. He was a recent tech school grad and they taught him that an ohmmeter was a gold standard transistor tester. He could not follow instructions on how to repair a power amplifier and many other things. He didn't read service bulletins. My recall rate was nearly zero. We had a number of units come back that he repaired. None of these would have come back if he had followed my instructions.

A couple years later we had a unit come in for repair with a note that an instructor at the above tech school said that I should replace a certain transistor to cure a noise issue. Er ... after a brief shot of Volume control cleaner the unit was fine.