The only legit argument is that you can install a very malicious app.
This would be bad if Google and Apple didn't already allow malicious apps in their stores already.
Google already has play protect, but it doesn't do shit. If Google legitimately made play protect work, then the risks would be 0.
All of the pros outweigh the cons, though. Can't even tell you how many times an app has updated and completely broke shit. Being able to sideload the APK to downgrade to a previous version is quite literally a lifesaver.
I thought Google already allowed it. I've been able to sideload on my Japanese Android 10 device with no issues. Did they lock it, or is it being removed by the networks/manufacturers?