At this moment in time, it's batting 1000. One child was treated (on a compassionate use basis prior to any clinical trial being started)resulting in a complete disease remission. Too soon IMO to call it a complete cure, but it sure is looking good.
The CLLS risk isn't GVHD, since that is mediated primarily by one thing - and hence easily edited out. The risk is what they have to do to prevent HVG from wiping out the treatment. In order to prevent that they have to lympho deplete much more significantly and for much longer than auto CART. Which is risky from an infection pov.
So in aggregate they are likely to have either lesser efficacy (because HVG wipes out their treatment) or high mortality from infection. But I'd suggest that one patient is much less likely to show this because they will get special care which wouldn't show up in a larger trial, much less if SOC.