I imagine if the workshop doing the tuning was also resposible for ALL of the mods done to a customers car and know the history of the vehicle especially if its been to them since new....then, one would hope the workshop would be almost fully liable for a popped engine during dynoing or even normal use.....
So the reverse - if you get mods done here and there and then go in to another workshop for a tune, they really would/should have only limited responsibility but still one should expect that "all care taken" still applies.