No it's not. And the latter doesn't happen. It doesn't.
The riMac runs hot.
Apple says it themselves. In fact, it runs hotter than the top-end Mac Pro at full whack.
However, the key takeaway is that it's not an
issue like many people make it out to be, it's just how it is. It's like the original rMBPs that hurt people's laps because they got so hot. Bad for humans, but fine for the hardware.
I'm looking at getting a 5K iMac too. I've done an almost obsessive amount of research on it, as always. I try to avoid subjective complaints, though. My view on the situation is as follows:
The fans are running harder on lower intensity tasks than previous iMacs, which is why people are complaining. Those who aren't complaining either don't notice fan noise, have room acoustics that dull the sound, or don't do many
mid-intensity tasks.
Remember,
high-intensity tasks would set fans off on old
and new iMacs, so you've got a niche of people who are specifically doing
mid-intensity tasks that will notice the difference in fans. They're frequently carrying out tasks that wouldn't set fans off on a 2013 iMac, but now do on a 5k.
A subset of those people are complaining.
Edit: If anyone else is better informed - please do let me know. The above is simply my understanding.