I hate to say it--because I love Demogorgon, and I love most of what I've read of MToF--but the more I look at it, the more I feel like Demogorgon just isn't up to snuff in 5E.
First off, he's among the least interesting of the Demon Lords in terms of combat. Nearly all of them have really interesting tricks they can pull. The only thing Demogorgon has that even leans in that direction is his trio of gaze attacks.
And those are made a joke by the whole "You can avert your gaze to automatically succeed" thing. Making a gaze attack takes up Demogorgon's entire action. It's not a bonus; it's not something he can do as part of multiattack. He looks at you, you look away, his turn is over. Yes, there are the various advantage/disadvantage issues I mentioned previously, but that's not really enough when dealing with one character out of an entire party.
"Oh, but he can still take legendary actions!"
Yeah. Two. Not the three that are standard to nearly all legendary creatures. Two. One of which is... Another gaze attack that can be looked away from.
Yes, as the DM, I can adjust. I'm very well aware. I'd still be happier if the Prince of Demons actually lived up to the name as written.
Yea, they way underpowered the BBEG's in there, with the exception of Zariel, or whoever that fallen angel lady is, she's pretty fierce. But the others, Bel for instance? He's got less than 200hp, but he's supposed to be one of the fiercest warriors in Hell, a major war leader? I ran a straight up fight between him and a hypothetical 15th Paladin version of the Paladin in my group. The paladin didn't beat him. He WORKED him in 3 of 3 arena style matches. I'm sure many will say that isn't how he would be played in a game, and they'd be right, but that's sorta missing the point. That, and also Prot vs. Evil is way overpowered for a 1st level spell
That being said, I really don't think they playtested the final versions much, or at least want 20th level Characters to be considered if not as powerful at least in the same league as Arch Devils and Demon Princes. But I am shocked at how little magic a lot of powerful demons and devils have at their disposal. It is hard not to simply see them as a brute squad. A standard issue Balor has exactly no magic at it's disposal. It's a fighter with wings. You have to customize them if you want some flavor. Me, I give them X amount of 3/day spells of level 5 or lower, and Y amount of 1/day spells of 6th-9th. What those spells are exactly depends on it's background. Balors that work for Orcus might have Contagion and Circle of Death, while Balor of Yeenoghu can cast Conjure Animals but it just makes Gnolls show up. The Planetars and Solars also have a purpose. The ones in charge of mortal revelations casts Dream, the ones sent to punish cities can cast Earthquake or even Meteor Swarm at will.
In 1e, Demogorgon was the meanest head to head monster of them all in my opinion. I believe that is his role whereas Orcus power is both personal but also organizational. I think anyone who gets within 10 feet of Demogorgon shouldn't live.
Of course, in 1e all Devils and Demons of any sort of rank at all could teleport at will so it is really hard to kill them if they have enough hit points to take a shot.