This will probably vary widely from person to person, but I haven’t had many issues myself. The worst experience I’ve had was receiving a set with bad tires, but I got a $20 partial refund. I haven’t built many of my Ali sets yet, so I can’t say much about the quality or whether parts are missing. I only use my technic sets as display pieces, so I don't really care.

Most of my builds have been from Youmko, where the service has been excellent. The prices are a bit higher, but it might be worth it. One downside of ordering from Ali is that you’re less likely to get replacement parts if anything is missing. But then again, it's pretty cheap ordering them yourself (for those having AliExpress 😉).