Temu doesn't actually sell anything. It's an online market place, just like Amazon MarketPlace, where lots of different vendors offer items for sale. So, inevitably (depending upon just how good or bad a particular seller is), some purchases might go through very smoothly, whereas there could be big problems with some others.
As Temu is based in China, it's likely to be a popular outlet for Chinese firms selling fake good or ones that have been manufactured using forced labour:
https://www.bbc.co.uk/news/business-65990529
With many of the items sold being sent from China, there are potentially problems with long waits and/or having to pay hefty import charges to get your goods from the postie.
However that doesn't mean that there won't be any genuine bargains on the site. One simply needs to exercise a certain amount of caution when using it.
I see that Light In A Box has been mentioned above. I've looked into it before and my own opinion is that I wouldn't touch it with an infinite number of the proverbial bargepoles.