The standard MCB/RCBO is too large to fit in the yellow brick, and they do not have appropriate weather proofing, the small thermal only trip does have a cover which can be used, and as far as RCD's go, there is a problem finding ones which will work with 55 volt, there seem to be some in line versions, as to if it really needs RCD protection for the extra 5 volt is debatable.
The biggest problem with on site 110 volt supplies is fire, and a path to earth is needed to get the 40 amp, so use of a RCD would help, not against shock, but from fire.
As to TN or TT it hardly matters the yellow brick is portable, so it may end up with either.
However although the danger has been identified, the problem is how can one fail a brand new device? If a safety officer bans them, OK, but as an electrician we don't really have the clout to say not to be used.
The 110 volt site rules do not apply where I work, however again talking about portable equipment, and is a tradesman buys 110 for one job, had to say you can't use it on another. Where I work water is the problem, a building leaking like a colander, and we also have marques and the like, which are lit for events, and the 110 volt festoon lighting is less likely to have problems out in the rain.
I do question LED in festoon lighting, as very little heat to dry out any water which does get in, I wonder if 110 volt is more aimed at making it so stealing bulbs is pointless.
The common situation is a long 230 volt lead feeding a brick which then feeds the lights, and one does wonder what is the point, they want the brick local to the user so they can remove the 230 volt supply, so the brick does not get left powered up, as even with no load they get quite warm, however that seems to defeat the whole point of having 110 volt.