Yea I've basically come to the conclusion that any one monster thrown at a party of 5 players is going to die. It doesn't matter if the party is level 5 and the monster is CR 15. The action economy is just THAT skewed in the party's favor. My group of level 5 players (Paladin, Bard, Rogue, Warlock & ranger, only one of whom is a Min/Maxer) can handle a high CR monster with absolute ease if they've recently had a long rest. It doesn't matter if it has 180hp and dishes out 3 attacks per round dealing 60 damage within them. If I roll bad on initiative, it'll be dead in the first round.
I bet you they cannot handle a well-played Adult Red Dragon with 5 levels of Dragon Sorcerer. Dragon strafes them from the air from within Darkness spell. Paladin cannot respond effectively unless the wizard casts Flight on the Paladin, but the dragon just Counterspells it, or casts Quickened Dispel Magic after the paladin has begun to chase him, causing the paladin to fall for 6d6 damage or more. Rogue cannot respond effectively because Darkness prevents sneak attack from working (because disadvantage--and the dragon has Blindsight to boot). The Warlock and the Bard can cast some spells for minimal effect, but the dragon has Legendary Resistance to make his saves. If the Warlock has Devil's Sight, he can at least ignore the dragon's Darkness to attack it normally with Eldritch Blast, which maybe will damage the dragon enough to make it drop Concentration... but Legendary Resistance works for concentration checks too, so probably not. And if the dragon hasn't had to Counterspell this turn, it can Shield for AC 24, which makes the Warlock's chances of even hitting in the first place pretty low. For everybody but the Warlock, hitting AC 24 at disadvantage is essentially impossible.
If the party miraculously begins to gain the upper hand, possibly after one or more PCs die, the dragon can drop Darkness in favor of Expeditious Retreat and retreat at 280' per round (including Wing Buffet), and come back under cover of Invisibility and his Stealth +6 (effective +11, probably beating any PC's perception) for a surprise round later, during which he will:
1.) Cast Quickened Darkness on himself (specifically, on an object he's carrying) to gain advantage.
2.) Breath fire on the heaviest concentration of PCs.
3.) Tail attack surprised PCs with advantage on their (surprised) turns.
4.) Laugh at the expression on their faces at the end of the surprise round.
But no DM should do such a thing to his players. Except maybe if it's a dream sequence intended to warn them not to mess with dragons until they are much higher than 5th level.
Edit: yep, I just tried this with my own 5th level test party from OotA. PCs failed to overcome dragon's Stealth roll (I didn't decide whether it was at night or in daytime because the dragon's Invisibility prevents it from mattering). I require multiple Stealth rolls for a successful approach, and the dragon rolled four times during its approach: 13, 15, 14, 12. Due to Darkness, the PC with the highest passive perception (16) had only 11, so failed to see the dragon's approach. I suppose in a real game with players I might have been merciful and ruled that that PC would have heard the dragon's approach without seeing it, preventing surprise and giving them a chance to stop and listen actively, but it wouldn't have mattered because he still wouldn't have seen the dragon (I just rolled four active checks for him and none of them were above 11) so the dragon would still have gotten the first strike in--and that first strike killed three of the four PCs outright and left the Moon Druid with only 1 HP. Not being surprised would have let the Moon Druid at least attempt to shift into animal form and flee, or fight, instead of just dying helplessly to the dragon's tail attack (18 damage) while surprised, but it wouldn't have changed the basic outcome of the fight, which was TPK with no real chance for the PCs to prevail.