"Buying a 2012, should I be worried?"
In my opinion, yes you should. To me, the reality is that the chassis that you want is the 2014 build (and onwards) chassis that has the welded in triangulation behind the engine. As has been pointed out by others, even these chassis may crack elsewhere (and be repairable) but now that the factory will not do a free of charge chassis change it gets very expensive to do the swap yourself or even to get the more limited repairs done where feasible. My view is that the 2012 chassis if actually driven is very likely to fail.
I do have some history here having had a 2012 M3W from new and have had the 2012 chassis, the 2013 with the additional small gusset plates and perhaps a different specification of steel tube and then two 2014 style chassis, one of which was doing fine till I hit black ice last year (and the side of a small mountain) which led to me having my current 2014 style chassis though I can't really blame the chassis design for that one!. The current purchase price of a replacement chassis is kind of painful and the number of hours to do the swap is not insignificant.