Not sure how the conversion was performed, but basically with a single wheel you have a smaller contact patch that is further away from the center of the bearing. Every time you hit a bump in the road it transmits more force to the bearing than the dual wheels (larger contact patch, centered over the bearing) which can lead to premature failure.
Unless I'm missing something that would only hold true if you continued to use a dually rim as your single outside wheel. Dually rim/tire loads are more or less centered right over the the rim mounting surface As long as your new single wheel rim stays with a reasonable offset, it should place no more stress on the bearings than the original dually config, because the load is still going to be concentrated in the same place it was prior to the conversion.
I dont know what Ujoints conversion process is either, but as long as he's been in business I can't imagine him doing a conversion, even requested, that he knows is going to lead to total failure. I've had customers request some fairly ridiculous stuff (non-automotive) and I've refused, because at the end of the day it's my name on that job and that is all people are going to hear. Failure, my name. No thanks.
If I had to guess I'd wager that was one of those "uh oh, got a big problem, gonna try to limp it home" situations that ended badly.