My eyesight is -7 so pretty bad. For offroad I use daily contacts and sunglasses for everything but flat pootles. Rain, mud, sweat and steam can be bad offroad and you need to be able to see the obstacles so I don’t want to rely on specs that I may not be able to see out of.
If I’m doing a short road ride used to used old specs but they didn’t have much coverage so my eyes watered a lots – so much so one of the tear ducts got infected and my eye watered 24/7 for most of last winter. I’ve now invested in some Adidas Evil eyes with RX inserts, I’m right on the prescription limit for RX inserts (the direct glazed limit is lower) so there is a tiny bit of distortion but get used to that after a few minutes. They are stupidly expensive though and I wouldn’t want to use then offroad if it was wet and gritty – for the reasons above as well as not wanting to ruin them.