We do still like the builtin Backup solution, as it allows our customers to see that there are in fact really backups made and available and also sometimes they use it to restore some stuff on their own. (but often it's like Hangover2 already wrote and we have to recover specific stuff for them)
We also have some big servers with multi TB data and hundreds of subscriptions and here the full backups takes 4-5h and the increments 20-30min
But these are all bare metal systems with SSDs for data as well as for the backup volume (that is local as well)
Though I wish we could get differential backups with the builtin Backup solution, as this would allow us to go to monthly fulls without the fear of losing to much data, should one incremental backup ever go bad.
As for a more proper/secure backup (these local Plesk backups are more for convenience) we use a Veeam Agent solution to get the data offsite and offline.
If the Acronis solution would allow own repositories and not force us to store the data "in the cloud" somewhere, we would maybe give that a try as well.
It's somewhat nicely built into the Plesk panel and due to being block based and "forever incremental", the performance should be pretty good, even with many TB