most of my campaigns run into the low to mid teens. I just kinda shrugged it off assuming wotc knew what they were doing & there must be areas that really make p for it whenever I noticed stuff. DiA however is so over the top slanted that it would not even serve as a believable white room scenario where nearly every monster has some combination of low ac, energy resist, energy immune, magic resist, legendary resist & it tops things off with being monty haul for noncasters with little more than a consolation prize & a few middle fingers directed at casters .I trust your experience. I am unsure how to quantify it. Googling, I couldnt find any 5e chatter about high level fullcasters being underpowered, not in recent years anyway. Likely, not enough players are playing the highest levels for the community to get a sense of where the class imbalances are. The potential imbalances wouldnt really start until, say, level 13 and higher.
There seems a consensus that unlike earlier editions, a 5e caster cannot go it alone. Because of concentration and fewer slots, the caster can only do one amazing thing at a time. Without a team, the caster is forced to multitask and flail. These kinds of discussions mention that noncasters also cannot go it alone, and need casters in their team.
I suspect 5e hit the caster with a nerf bat too hard, with the intention to end "quadradic Wizard, linear Fighter", once and for all. It might even be that 5e has now become "quadratic Fighter, linear Wizard", such as the Fighter multiple attacks multiplying increasingly high damage attacks.
DMs and players who are noticing class imbalances in their own high level campaigns need to start getting a handle on what exactly is happening, before a fix can become possible.
It was so bad to watch as a gm that I started looking at the math. Flatly had trouble believing it at first glance though... That lasted until I decided to play trough it as an artillerist & make this spreadsheet. From DDB's numbers something like 90% of campaigns end at or shortly after level tenish so my low-mid teens average campaign end is well outside the norm so it's not surprising that there aren't many complaints about it.
Casters in earlier editions couldn't really "go it alone" though. Arcane casters were at serious risk of lie & limb from nearly any monster from zombie up. CoDzilla was kind of a special case that required a player optimizing for that specifically and a gm who didn't do anything in encounters to counter it or give out magic items that brought other PCs up. I know exactly what is happening and can't fix it without making a whole new heartbreak rpg type fork of 5e because the system is designed with too many layers designed to fight against easy corrections like magic weapons used to offer when LFQW was a thing they actually countered.