Both. It's a good cartridge, but it has some baggage. It's relatively new (1991 - and, given the very low volume of the double rifle world, that makes it very, very new), and it's also the highest pressure of the over .375 double rifle rounds. Brass and factory ammo are not as well distributed as the others. I think the primary problem with the .500/.416 is the widespread perception that it kicks just as hard as rifles of the same weight in the .450/.470 group, so why not just go ahead and get a .450 or .470?

The .400 doesn't suffer from that. The gap in double rifle cartridges between .400 and .450 existed for 90+ years because the .400 was so good, and that's also why it was the most popular of the over .40 DR rounds. Now that Hornady has introduced .400 ammunition in addition to Kynoch and Romey, and we have Krieghoff, Demas and others following Heym in building rifles for the cartridge again, I think the .500/.416 will become an even harder sell.

You got it. Again, it's a matter of perception. As the cheapest method of barrel construction for double guns, mono-bloc has been used to make a lot of what fails to rise to the level of cheap junk. It's also been used to make fine double guns. Done well, the only difference is the visible seam (it can also be done seamlessly). As the price rises though, that seam can become unacceptable to some traditionalists who expect different construction methods for the price point (guilty), even though mono-bloc isn't technically inferior. That's why Heym will do it both ways, and they're not the only DR maker that offers different construction methods according to what the customer prefers.