Well, I have a MX5 that sheared the output shaft, and in the search for a replacement found the gearbox used on the 5, 6, and 7 series were all the same--also the same were formerly used by a company called Tiger out of Canada on their batwing mowers.
I suspect there are a lot more 6's out there than the other two (single spindle 7's don't seem too popular for any shredder line). I have to admit I have not seen a published failure regarding MX7 or Tiger mowers gearboxes, but given that the output length and gearbox design are the same, I would consider them to be at risk for the same episode. I replaced my busted spanish JD (Comer) with the updated Chinese gearbox (also from Comer) that uses a castle nut instead of a bolt for attachment.
The output shaft is definitely more robust now, but to reuse an MX deck you need an output that is 50-100% longer than a typical output shaft is... I could not find anyone but Comer making one that spans the distance.
The MX's use two sheets of steel to keep the topside free of dents (except the front 25% or so) and do all the damage on the underside, and ties the two together using a steel bowl. The spacing from the top of the top deck to the bottom of the steel reinforcing bowl (that spans and ties together the decks and mounts the box) are why they are so hard to find a replacement for. I don't think batwings and larger mowers tend to have quite as varied a diet as the smaller shredders get.