Moloch's a nasty piece of business, if handled well by the DM.
Enter combat with fly already active. Drop the DC 21 fear breath on the primary fighters. Then start splitting the party. Teleport 120' away from any un-frightened PCs at the end of the first player's turn using a Legendary action to avoid attacks; teleport back in and start dragging away the party healer with whip attacks to separate him from the rest. Keep using a combo of whip attacks and teleports to bounce all over the battlefield; forever remain out of reach of the barbarians, paladins and melee fighters; use Legendary Resistance / Magic Resistance / High Save Bonuses to ignore the spellcasters. Any PC with low Con saves gets a DC 21 stinking cloud as a Legendary action, effectively stunning them. Ouch.
If all else fails, get a big round of attacks off... and then Moloch uses his next 3 Legendary actions to teleport 360' away. Out of range of any radiant attacks, he regenerates 20 HP per round until fully healed - then resumes the assault.
As long as the DM uses Moloch as a hit-and-run assailant, I could see him absolutely wrecking a typical 16th level party.
Nice analysis. My first thought was he is under powered (which he is in a way), but I did notice the 120' teleport legendary action.
Also Drizzt's influence in 5e in very clear, it shaped one one, but two archetypes, Hunter is clearly taken from Drizzt's fighting style and alter ego, and Beastmaster is inspired by his relationship with his Panther companion.