Yeah, caution and skepticism are best practiced when shopping via aliexpress. you will get charged vat etc but its unlikely to find its way into hmrc hands. but you can save a few quid .
The sellers can be in china, russia, and other places you may not want to send money too.
And if its priced like a fake it IS A FAKE, and dont trust batteries from there even rechargable cycle lights will probably run out of juice half way down the rd, mine do.. tho the £2.31 pannier bags are still going strong not even a zip failure (yet)
for example if an aliexpress ad for a kt controller fails to state its a sine wave controller you can virtually guarantee its a cheaper square wave controller while more local suppliers may not even stock the square wave variety
Generally, it's OK and a good source of cheap stuff. I buy loads of things from there. The listing says if taxes are included if they are, which is now many of the items. If it says shipped from China and no mention of taxes, then you might get a bill when it arrives in the UK, but generally only if you paid something like more than £100. Small items, like controllers never get taxed. batteries often do, but not always.
Use Aliexpress if you can't get the item from Ebay. Often the Ebay price is the same, but taxes are already paid, so no hassle.
Bought a few things off AliExpress - battery mount, hydraulic brakes, tools, Bafang rear hub kit (came from Poland but was significantly cheaper than ebay)- all been fine so far - no duty to pay so far. Try to use trusted retailers, topbikekit, greenergy etc where I can
One can get some quite good prices on Ali Exp, though one may have to hunt for them amongst the many 10's of sellers.
I was after an eyepiece for my telescope , the UK price was £89 from most of the usual optical sellers. I procurred a new one for £57 delivered via Ali Exp.
AliExpress is OK for some things IMHO, for example I have bought quite a few spares for my CR6-SE 3D printer, most recently full hotend spares for £25, crazy cheap (bought two), at least 50% cheaper than most other sources.
I recall @billyboya's AliExpress bought battery, which may well have coped for a longer period of time on a less demanding bike... but his eRider moped killed it quite quickly, and he never got a refund.