They should be reliable but sometimes they prove to not be. The problem is inconsistency during assembly. Aftermarket turbos can't guarantee how much power they make either since it varies by setup. But even worse, some of the turbos aren't mass tested so think of the manufacturer's quote as an estimate.
As long as the manufacturer warranties it, you should be good but remember that they will never warranty installation costs or down time. There were some complaints against street toys of poor welds and turbos that leaked and blew up. I don't know how they turned out, check out the thread on tdiclub and you can see the complaints. Garrett only sells 1 VNT 17 upgrade so all the bolt-on turbos from other sellers are all hybrids. You could get other stock turbos to fit but they all need some minor mods to fit.