This sounds like a mix of problems to me but mostly with Synology. Sure drives have built in thresholds and 3 years seems short to be but mostly this seems to be Synology and less so with WD. I have 8 WD Red NAS drives that I pulled out of service with 4.5 years of power on hours. There was no smart warning for power on hours. I have 8 other WD Red NAS drives running 5 years and again no smart threshold for power on time. Of course I'm not running Synology.

This almost seems like Synology is playing the enterprise SAN/NAS game of wanting you to toss your hardware every 3 years. Maybe there's a little be of fault here with WD but if so it's because that's what Synology wanted and not a issue with WD NAS drives.