For some reason, I’ve repeatedly had lenses made to a given prescription and not got on with them. This isn’t an online vs bricks-and-mortar thing; I’ve ordered two pairs from a high street retailer before and one have been great and the other I can’t get on with at all. Overall I’ve had three prescriptions and maybe ten sets of lenses, and only about two pairs of lenses have really felt just right, despite everything having been tested multiple times to make sure they’re an optical match for the prescription.
Buying lenses online is fine if you’re confident that you get on with any lenses made to your prescription. Buying frames online, again, is fine if you’re confident that you’re going to get on with the frames.
Last time I had an eye test I had some frames reglazed online to the new prescription (which was significantly different to my old one) and the result was way off, but I’ve got no real recourse here because I went elsewhere for lenses. Personally, I’m now contemplating forking out for yet another eye test so that I can buy lenses or glasses from the same place and put my foot down if they’re not bang on.
YMMV, I think my eyes are just weirdly fussy.