Is App Cleaner by Nektony good for my Mac?

No, nor is any other so-called cleaner application. They tend to remove valid components from applications and the operating system and in some cases, that has led to reinstallation of the operating system.

I use their App Cleaner & Uninstaller for several years and I am absolutely satisfied with the quality of the app work.

The point is that when you just drag and drop the unneeded app to Trash a big amount of service files stay deep in your system. If there are a lot of such files, Mac eventually stops working well. This app does nothing more than just gather applications` service files and helps you to quickly remove them along with the executable. so I don't see anything wrong with this software.

Moreover, I can say that with this app my life has become much easier. When I just bought my Macbook I tried at first to manually search for service files or delete them using Terminal. But there was always a doubt that I can delete the necessary file for the Mac to work correctly. in a year of working with App Cleaner & Uninstaller, I have never removed anything superfluous. The plus is that there is an option to double-check all selected files before deleting them.

There is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This user tip describes what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community   

The point is that when you just drag and drop the unneeded app to Trash a big amount of service files stay deep in your system.

You've certainly bought wholeheartedly into the FUD.

The only apps that will leave anything of particular impact are ones that have an installer. If you must use an installer, you would just use their installer to remove.

Regardless, nothing of particular impact is "deep in your system." The only locations you need to worry about are LaunchAgents, LaunchDaemons, and Extensions, all of which are in one or more of the Library folders. None of which are anywhere "deep" in the system.

Yes. App Cleaner & Uninstaller supports 1000+ apps to completely uninstall from Mac. Unlike other applications, it finds all support files of applications for safe removal.

You should use the uninstaller provided by the developer of the app.

The only time you would need one is if you used an installer to get the app on your Mac.

If an app requires and installer, but doesn't provide any instructions or an uninstaller, I wouldn't install it in the first place.

One can use these useless #$%^ applications as long as one understands - use at ones' own peril.

A complete Erasure of the Drive & Clean Installation of macOS would qualify but do not think this is what is really requested ??