I haven't purchased any Shimano components through AliExpress, but I'd confidently buy ones if I couldn't find them in my country.
From what I know, AliExpress is against fakes. Sure, you can find fake frames, etc., but the key is in how they sell them.
If they state that it's a Shimano R8020 groupset, you can be sure that it's a legit Shimano R8020 groupset. If they don't mention the brand and the model even though the pictures show that they look like Shimano R8020 groupset, avoid them.
If you look at fake frames sold there, they won't show the complete picture, and they won't say the brand.
Also, better to look at those which have the most purchases.
If you received a fake when the seller had declared that it's not a fake, you can report them to AliExpress. You should be able to receive a refund.
Edit: here's their rules for sellers regarding counterfeit items:
https://sell.aliexpress.com/en/__pc/77Y4QdcvjD.htm