currently 7 (have had more but some left over time). Every single one has had some sort of issue out of the box or within the first 200 plays which took the game down and needed either parts or major tweaking and fixing to get it to play correctly.
4 of them have insert ghosting currently (KISS, SMVE, GB and minor on GOTprem).
I honestly think your reliablity perspective is a bit off based on your earlier WoZ experience (understandably), but my impression and experience is that TH (and the more recent built WoZ) are pretty damn solid with most issues addressed and TH has held up well with more plays than most Sterns and the plays are more steady also.
Trust me, I fought against getting a JJP due to price alone, but in my experience it has fewer issues and is played more. As Stern prices actually grow closer to JJP the dollars/maintenance/earnings/headache/reliability equation starts to look better and better for JJP. We still do this 100% for the fun factor and buy games we want to play and share, but the decision is getting easier to buy the higher quality JJP products while the risks get larger with buying the poor QC and quality Stern products.
You have to also factor in resale value and we have already seen the new Sterns taking bigger hits and buyer specifically avoiding GB with any ghosting...