I'm so sorry to hear about your losses. They are particularly heartbreaking when they happen as a result of a decision you have taken, even though you made it in their best interests.
I have a different take on things and know that neutering with an experienced vet is not always trouble free. My vet carries out hundreds of piggy neuterings, for example he does all our local RSPCA piggies and I've spoken to them and they have never had a post neuter death.
However, even with the most experienced vets, the worst can happen. I love boars and have had four neutered. The first was carried out by a general vet, before I knew about exotic vets, and was completely straightforward. The second had an abscess which thankfully healed easily. The third and fourth piggies were both OK in the immediate period after their ops with the most experienced exotic vet at my practice but (like yours) both went downhill a few days after the op and both died. It was not clear both times what had caused their deaths. I even had a consultation before the fourth piggy was done to discuss the risks and the vet said he'd just done a batch of 40 RSPCA piggies without incident.
I know it's just bad luck, but I would not personally neuter any boars after my experience, it's just too heartbreaking. Remember you did what you thought was best for your boys out of love for them and are totally blameless. Don't let my experience put you off if you feel neutering is the best thing for your piggies. Using an experienced vet is definitely the best way to reduce the risks involved.
Big hugs