Good information about the grounding points. Thank you.
I've never been happy using chassis grounds for my own accessories. I'm not entirely sure why that would be - It must have been something that caused me problems at some point. Whatever it was - I have a single additional 30A black lead for connection to the negative post of the battery, and everything that I install has its own earth lead spliced into that.
Personal hang-up maybe, but don't forget that all connections to the chassis do not terminate at the chassis. They terminate at the battery negative terminal - so if either the battery's ground point to the chassis or the battery cable / ring terminal are not up to the job of returning the combined current of the normal bike, plus the accessories, then there may be problems ahead.
There is no issue for adding a few small extra devices (3 of the fused circuits used by the quartet harness have some spare capacity, for example), but start adding heavy current extras (eg heated gear 5A each; a loud horn - 18A; extra driving lights 3-5A each; etc etc) it all mounts up. Earthed to the chassis or not - it all has to get back to the battery.