Ah! I had misunderstood the earthing arrangement. Couple of points though...
You say their main problem is nuisance tripping, yet I have observed that happen very rarely. Compared to RCDs they look like angels in this respect. The RCD is vastly inferior when it comes to nuisance trips.
As far as safety goes, v-ELCBs may not add a danger but they are so inferior to RCDs that they do represent a risk relative to an RCD protected TT system. TT systems are somewhat different to well earthed ones, and do need some extra protection because of this. And a v-elcb only goes so far in this respect.
Regards, NT