1. Incredibly fast and easy to install compared to pi-hole
2. It's easier to update because you don't have to ssh into the raspberry, you can just update the thing through the user interface.
3. From time to time, it happened that the pi-hole hanged up, dns resolution did not work and I needed to reboot the thing to make it work again. I am not sure how widespread this is but I've seen many other users complain about this particular issue (even though it's a once-per-month thing).
(4. Better APIs)
For an in-depth comparison between the two you can take a look at the AGH GitHub Page[0]
[0]: https://github.com/AdguardTeam/AdGuardHome#comparison-pi-hol...
This has the advantage of working on any kind of device that allows you to manually specify a DNS server IP address, without having to install or maintain software.
For example, AdGuard maintains public DNS server IP Address options that: filter nothing, filter out ads and trackers, or filter out ads, trackers and adult content.
Hardest part for me in the set up was that ad guard should query my router for local domain, because that one keeps track of which dhcp IP address is owned by which host. (This way I can always use host names on my internal network even for devices that get an address via dhcp. Very convenient if you play with Pi Zero and other toys)