I had the same concerns before I fitted my cheap Chinese import too. But I needn't have worried. The supplied tape has been strong enough (18 months after fitting, no issues). I used some clamps to ensure a tight fit overnight as one (the right) side seemed to have a bit of lift in it initially. Obviously I'm biased because I've fitted one, but I think the car looks miles better with the spoiler. I passed a Veloce without one yesterday (a rare thing for me!) and it looked like it was missing something to me from the rear! As others have said, ensure surfaces are clean, and warm things up! I used some kids plasticene to build a frame to slot the spoiler into - didn't like the idea of getting the positioning wrong!
Yes, there is a slight lift on the right. I couldn't live with a gap and am worried, that once stuck, it's there for life - although I'm sure there is a removal technique if it came to it. It does make the rear end look much more beefy I must admit. If it was a 100% perfect fit that you couldn't get wrong, I'd be more confident!