Our's is a 2011 frame from new and we've done loads of day rides plus there or four tours with the bike fully loaded with front and rear panniers and rack bags etc – probably around 60-70lbs. Neither of us is particularly large – our combined weight is about 285lbs, so we are looking at an all-up-weight of some 355lbs. I fitted the reinforcement bracket around the join (we rarely split ours) when we received it from JD Tandems and there was no evidence of any failure at that time. We've also had the frame stripped and resprayed and that probably would have bought any failure to light.
I only know of about four or five frame failures from accounts online, so I think they are extremely rare and knowing Hase's excellent after sales care, would think they would replace, even on an old bike. The serial number is under the seat on ours, on the right side flat part of the support.