If used with portal exit, thousand needles and spike trap, it can either be devastating or with stability completely harmless...
And since thief is 100% reliant on utility skills then if it doesn't hit, you are done for. Which from my experience using this alot is about 10% effective.. shame really cause the effort to pre-set this up should be better rewarded... say 50% effective.
But this is thief all over... 110% effort / 50% effectiveness.