my iMac mid 2010 nearly catches on fire when I'm editing and this is just with premiere pro being open. As far as overheating and running hot. There is a difference. But, as stated by hozzie. When your computer is running that hot almost too hot to touch, you then question the longevity of the product. (which in other words is overheating).Yes the only time I have heard the fan kick in on the 5K is when FCPX is rendering 4K footage in the background but running hot is different than overheating. If its within tolerances then let it run hot.
I have to admit the only thing I was nervous about was the AMD GPU as Apple had those GPU failures on the Macbook Pro with the AMD chipset. Apple eventually agreed to replace the GPU on those out of warranty but I did purchase a 3 year Apple Care on the 5K. Considering the money you drop on the 5K the Apple Care is a drop in the ocean. With that you have the insurance you need.
In terms of Adobe, I dropped from the full subscription down to the Photography subscription and use FCPX for video. I used Premier Pro for sometime so know the differences between the two. FCPX is pretty good.
As far as switching to FCPX it won't happen for me as I become very accustomed to After Effects and I'm just getting my hands wet in Special effects. For me, Adobe is a must. I'm very sickened on their subscription base now.