I'm not a fan of slan TG combo, but I must admit it is very very resilient, in addition if you get hit by a purple sum, or the pit, remember slan gets look out sir, because it's a template. And as Arli mentioned with becalming cogitation even the book of hotec/dweler's isn't that scary (they still have to roll 21+, and with the 6's gone it's difficult.)
Another thing the TG+slan is ideal for a life slan, it's not an auto win, but it forgives plenty of mistakes. In addition you dont have to worry about thing's like flank or rear charges. Last tournament I played (came first) I could have afford to pursuit with my TG, knowingly I would receive a rear charge by a huge unit of irongut's with tyrant, and possibly a flank with a yeti or gorger. I knew that I can afford to lose combat and have a 98% chance to stick around, do a combat reform and win the next combat round. Not something I could do with the saurus. I love the lone slan, because of maneuverability, but in a competitive environment like a Tournament risking your slan is something I'm not willing to do. And that is why a BSB slan in a unit of 25 TG is soo good. a fluke or a bad dice rolling is minimized to less the 2%, it only get's better if your unbreakable.