SSD for Mac Mini 2012 -- Need Second Opinion
There is a small element of historical truth in their recommendation. Until quite recently (10.10.4), the Mac didn't support TRIM on 3rd party SSDS. That meant that in some use cases with some SSDs, the drive could theoretically wear out faster and suffer from performance degradation over time. The practical effect was always hotly contested but it doesn't really matter since TRIM for 3rd party drives is supported in Sierra. 

There's no obvious reason that your WD Blue SSD wouldn't work

A Fusion drive is a bit trickier as you have to completely disassemble the Mac Mini to install two drives in it. Replacing a single drive isn't too bad as long as you have the right tools, a reasonable amount of dexterity, and are good at following directions. There are a couple of tricky bits and sliding the drive back into the right position can be frustrating but nothing too bad. I've done it about 10 times without screwing anything up. One SSD failed but it was after 5 years so I doubt it was related to the install.

Macsales is a good source of the right tools, adapters/parts, and instructional videos. They also provide good tech support if you buy one of their drives.

Either look for another shop that is more comfortable doing the upgrade, tell the current shop that you appreciate the advice but you want what you want, or do it yourself. For a simple replacement, MacSales sells the toolkit for $5.

https://eshop.macsales.com/item/OWC/TOOLKITMHD/