1
UZR is still really fuzzy, but DER is prone fluctuation as well since its very BABIP impacted. Think of UZR as OBP. OBP is going to be prone to fluctuation year to year, won't give 100% accurate reading of a team's offense, but at the same time still gives you a snap shot of a guy's defensive performance taht season.
I absolutely agree that group stats are more accurate if you're looking at stat on a team level (though they don't consider XBHs), but UZR has it uses along with Dewan's plus/minus, PMR RZR, OOZ, etc. I like Dewan's stats the best since they skill base defense so you get a better picture of a defender strengths and weaknesses.