While a pool newbie, I am an old hand when it comes to plastics.

I would be very surprised to see/hear of a plastic liner cracking because it is 28 mil in thickness. More likely, if the word of mouth were to be believed a bad batch of them made it out of the factory which had the wrong amount of plasticizer mixed in with the plastic (or didn't have an adequate UV inhibitor). Both would cause them to crack when under stress. One likely during the actual installation or shortly after filling while the lack of a UV inhibitor would cause it to break down more rapidly once it is exposed to sunlight.

That said - when it comes to strength...all other things being equal, thicker is better. You should get a longer life out of the 28 mil liner than the 20 mil liner. If the first guy installed a liner 12 years ago - I would consider him to be fairly well established now.