Are they effected by low g like teethering head helicopters.
Yes, Yes and Yes again!! In the 70s/80s they became quite popular (possibly on the back of Ken Wallis's 'Little Nellie'), especially home-builds. The majority of incidents arose from inadvertent or intentional entry into low G territory. a definite no-no. They can be safe - viz Ken Wallis. Unfortunalely, they are 'fun-machines' (and much marketed as such) - a potential problem for the unwary!