Just got a reply from them, and im definitely never giving this company my custom again.
"In this matter, as a point from the manufacturer of the internal hard drives, as far as WD is concerned, our EFAX drives are capable of running in NAS systems with no issues. However, please bear in mind, every NAS engineering is slightly different than one another, therefore since the compatibility is depending on the Host devices, you may contact your NAS manufacturer for your future inquiries regarding SMR technology drives."
They then linked a list of NAS which for some reason doesn't include the DS918+ i use, even though on Synologys own website it lists the drives as being incompatible.
Now theyre asking for proof of purchase and performance issues experianced etc.