I didn't say the game is "Bad".
I said the game is inherently imbalanced because the original designers didn't take into account the potency of novas and the 5 minute adventuring day.
Based on Mike Mearls, one of the people who designed the game in the first place, openly admitting that they'd overestimated how many rounds of combat players would be in during the day by a factor of 5.
There's at least two ways to fix this problem. Either inflate the hell out of NPC HP in order to make them survive long enough for the nova to be impactful but not end the encounter right away. This has the side-effects of both destroying CR calculations, and encouraging a long rest before every single encounter even harder.
I'm suggesting a different way that addresses the issue, directly. One that would make CR calculations actually -work- as they've been balanced.
Better idea: throw out the whole notion of CR.
There's just too many variables that the designers can't account for to allow any such system to work well enough to bother with:
--- party size; not every table plays with 4 or 5 character parties at exactly one character per player; some have parties of 2 or 3, others run parties of 6 or 8 plus henches and hirelings
--- party composition; a well-rounded party can take on a lot more than can an all-Rogue or all-Wizard party
--- optimization levels and-or acceptance at the table
--- amount of, and specific details of, magic items owned and-or being used by the party at the time
--- specific environment and terrain features present when the party meets the monster (a dragon in a confined space e.g. a cave will present much less challenge than it would outdoors where it can fly)
--- synergies or conflicts between monster abilities and-or environment (a monster that breathes cold will be far less effective in a very cold environment as the PCs will have already taken precautions against cold)
--- how increasing or decreasing the number of monsters changes things (4 dumb Orcs working together present very little more challenge than does a single dumb Orc, but 4 smart Dragons working together present a vastly greater challenge than would a single Dragon or even 4 Dragons one at a time)
--- dice.
Note that all of this has been true ever since D&D was invented, it's not just a 5e thing. 1e vaguely grouped monsters into levels, and even that didn't provide much if any guidance as to just how much of a challenge a given monster would represent right here, right now to the party facing it.