Obviously the correct answer is the useless one that it varies package to package. AUR is a community effort.
In practice, I use a great many AUR packages and they work just fine. I avoid the AUR if there is an alternative in Core or Extra but much of the value of Arch is the AUR.
The number of AUR packages is not really a factor. You can have dozens of AUR packages installed without incident. A single poorly ( or maliciously ) crafted AUR package can cause problems.
Dependencies can be a problem. I used an AUR version of GIMP for a while ( 2.99 ) but it depended on GEGL and, at some point, the version of GEGL was not new enough and it broke. Overall though, issuers have been rare in my experience.
If you do have an issue, fixing it is typically easy. Arch package management is great in my experience.
I would stay away from Pamac ( from Manjaro but in the AUR ) and just use yay. Pamac breaks things. If you want more than that, try Pacseek.