Good balance between UI and functionality.

PRTG does not protect itself from its own configuration. If you are creating ping sensors that attempt to ping too often that it can collect the results, it can have repercussions, such as grinding the entire system to a halt. PRTG also has a matrix about how many of certain types of sensors you can create on a probe - however the system doesn't have any kind of tracking for it, so the only real way of telling that the probe is overloaded is that it starts slowing down - and this impacts the data collection. The higher importance of your PRTG installation, you will have to have a keen eyed administrator.

Although PRTG is capable of scaling upwards, once you start to manage thousands of sensors, you need to start rigorously planning your probe deployment. The probe limitations may prevent you from logically grouping your sensor groups at a top level. And you won't notice performance problems until your data collection is already being impacted.

Our internal users use AD authentication. PRTG has a control that if there are too many failed logins in quick succession, then it delays all logins to protect itself from brute forcing. Unfortunately this does more harm than good when someone's password expires and PRTG is still running on a device somewhere (this happens quite often).

You can set a desired availability percentage in your settings, which applies to reports. Unfortunately, it's one setting for your entire environments - we want to monitor different availability levels for different services that we monitor!