5e at high level sucks because WOTC didn't bother to actually test any of it. They admitted they didn't test past level 10 and their inclusion of magic items is so bad that the recommendations for how many to include show the devs are terrified of them; you get so very few by official recommendations.
Save or Suck spells make higher level 5e (and 5e in general) a huge PITA to plan encounters for when players can just get lucky and end things outright. Magic items don't have a power level listing, just a rarity doing double duty that is wildly inaccurate. Class balance is shit: martials are boring. All they do is swing the weapon most of the time and very few class abilities really alleviate that. Spellcasters are so vastly more powerful and fun that multiclassing is way more popular than it should be.
Monsters in 5e are boring. Most of them are just bags of hit points that swing. Very few have bonus actions or reactions and the ones that do are often just "parry: increase AC once". They came out with gem dragons 2yr ago and they gave them all the SAME bonus actions: shapeshift and misty step. It's like they don't even bother to try, which is evident in recent releases.
/rant. But seriously, I love high level D&D. You can really raise the stakes. I'm DMing a level 12 5e campaign now and my primary way to make encounters interesting is two-fold: firstly is 3rd party content to get monsters that are actually interesting. Secondly I create objectives that aren't just "murder monster". I just had a bunch of literal street children hit my party with a net trap and arrow that makes them drop their items that said children tried to steal and run away with. 2 got away but they were able to find the hideout and defeat the boss. Actual stakes besides just getting killed.