As a vet, maybe I can shed some light.
The problem with wethers that are banded early is that the testosterone has not yet been produced by the body as the animal hasn't reached puberty. This means that the urinary tract develops a bit less than if it had testosterone, and therefore the tube would be needed to transport all the manly juices. Sometimes this means that the urethra is narrower than a male that has not been castrated, or has been castrated after puberty. If this is the case, it is important to carefully manage diet, as high concentrates can predispose to forming crystals in the urine which can get stuck in a narrower tube.
This is why some people castrate later, once an animal has hit puberty, by cutting the scrotum and removing the testicles, or using a burdizzo to crush the cord. Either of these procedures is a more severe procedure than putting a rubber ring on at less than a week of age, so there is more risk of the animal having complications, such as infection, bleeding, or death. Also, (certainly with lambs, I'm not entirely sure about the exact age for kids) any lamb over 3 months of age being castrated must be done by a vet, with appropriate analgesia (pain relief) and anaesthesia (either local, or a full general anaesthetic depending on the animal and the vet). Again, adding in the anaesthesia increases the risk to the animal.
I personally think that the banding method (which I have used over many years when lambing in commercial flocks, and for my own lambs) is quick, fairly painless and fairly low risk. The risk of urethral blockage later down the line is a very rare complication, and I'm so sorry for your loss, but it shouldn't put you off castrating.
And ScotsGirl, I appreciate you were worried about the lamb, but I wouldn't recommend cutting off a band, unless it is immediately after placing because the placement was wrong. The bands cut off blood supply to the scrotum and testicles, which means that the area with no blood supply starts to die back. This can release a lot of toxins, which could be released into the bloodstream if the band is cut off. This can be fatal, so I'm most relieved that it was only his testicles you lost

Hope that helps
Suzanne