What's the difference OEM VS OEM-compliant

• OEM Flexes are thicker and more resistant to tears and have better quality soldering for the connectors.
• OEM glass will be stronger and less brittle. OEM LCD's will have better colour response, viewing angles etc.
• Most importantly, you want a cold pressed glued (CPG) versus hot press glued screens as they will last longer before the glass separates from the frame.

You have to read between the lines on most of these vendor websites. They use words that normally mean high quality but in a context that means anything but. OEM is original equipment manufacturer. That means that whoever builds an actual part for Apple products. They have certain quality specs to achieve in order for the lots to be approved. If they are also manufacturing similar parts, they could claim to be OEM parts but they might not meet the same specs.

Everything has a price, you just have to pick based on your needs.