I previously ran a PC with the software, the Ubuntu install was manageable but later versions 19.x a bit of an issue, I want a high performing network not a Linux hobby farm so decided to buy.
Pound for pound the cheapest way to run your UniFI network and has great features too. I've run the UniFi controller on a Pi and a PC and in both scenarios the amount of faffing about time AND my electricity saved pays for this device. 750 watts of PC on 24x7 or 6 watts in this unit, easy answer.
Decide how-much you'd pay yourself / earn per hour and divide that by the cost of the unit + electricity and if your reading this on Amazon AU website you'll know the cost of power.
The features between the downloadable version vs this appliance are mostly the same but upgrades / updates are a breeze.