I'm starting my capstone encounter this Saturday for a campaign I've been running since November 2018. The PCs are levels 19 and 20.
In my first D&D campaign, which lasted a little over a year, starting in late 2014, I ran it to level 20 but was using milestone leveling.
The advantage of playing a long campaign with slow (for 5e) leveling is that as a DM I get to know the players and their PCs quite well. In my first campaign, I often found myself ill prepared for what powerful PCs can do.
My main issue with high level play is that most published material I've found just isn't a good fit for my players, who are all long-time, highly strategic and tactical gamers. Most of the "high level" material I've looked at my players can steamroll at the recommended levels. That means that I either have to create the encounters myself or highly customize them. It requires a lot more prep work on my part, which is difficult to keep up with with work and family commitments.
In my experience running 5e, the higher the levels the more important it is to know your players and your characters and it is very very hard to develop a high level adventure that will work for most groups.
Also, unlike my players who have literally had years to master their characters, I find it exhausting to run combats at high levels because of the number of abilities and spells you have to keep track of. I find I have to spend quite a bit of time thinking and planning on the enemies strategies and tactics and develop playbooks of sorts for high level encounters.
I still like high-level play, but in the future, I would likely go for slower advancement at lower levels and do capstone leveling at higher levels. I like the craziness of high-level combats and challenges, but I need a lot more time to prep them. While I would lose the deeper understanding of the PCs, I would have more time to prep the enemies.