Well, the factory service manual claims the 400F has a "pointless" regulator... but it actually has a standard electromechanical one with points. So maybe the advice is to protect the nonexistent electronic regulator?
The diodes in the original rectifier have a rather low PRV rating, the maximum reverse voltage they can handle. It's possible some external chargers could blow them. No charger designed for our small motorcycle batteries would be able to do that, but automotive ones with high output capacity could.
I've used chargers and boosted SOHC4 bikes from car batteries many times without any problems. If you connect a car battery for a boost backwards though... expect smoke.
As for reg/rect units, the internal components are not expensive and the cheap ones may be great but may be really cheaply and poorly made. What you get with the expensive ones is some certainty it will work, and a warranty that, as a rule, they will honor - that gives them an incentive to make a decent product. An ebay seller has no such obligation.
FYI - it's actually very simple to make your own - a replacement alternator regulator from an automotive store, and a simple 3 phase bridge block from Digikey or Mouser.
