It's not a problem with the 32E. It turns out to be a very sweet setup, the 6 volt armature with the 12 volt fields. In this configuration it's virtually unburstable. My oldest conversion dates to the early '80's with untold thousands of miles under its windings. You're right though, I wouldn't do it to a Lucas long or short 6Volt genny.
There is no free lunch of course, the down side to the 32E conversion is that the cut in rpm's is slightly higher for 12 volts than 6. With my mix of town and country driving, it's never been an issue, I run 100w halogen headlights on all my fleet. (knuckle,pan,two BTSVs)
Here's the plus side:
The 32E 6 volt armature has heavier gauge windings than a 12 volt armature, whether a rewound 32e armature or a 65A armature. Heavier wire means less resistance, and therefore less heat given off by the armature when at work. And heat is what kills an armature.
So, the 6v arm/12 volt field genny has a better potential for longevity than a 65A or a rewound 32e 12v Arm/12v field genny.
I've proven this myself too many times with the 65A that was on my Panhead for years. On that machine, I gave up on 65a's and I went with the fan cooled genny with 12 volt fields and now have extra juice to run "HEATED GLOVES". Yeah, decadent, I know.
Long and short, if all you want is reliable 12 volts, take the easy way out. 32E, 6 volt armature, 12 volt fields.
Of course, you're always able to "pays your money and take your chances"