They do not load the engine as much as real blades; at least, none of the commercial offerings I've seen do.

What they do accomplish is to allow you to get your mixtures in the ballpark, check for engine-induced vibrations or radio glitches, and generally check out a new or repaired machine without worrying about dorking it.

Safety? Model helicopters are not "safe", period. If what you're really asking is there a potential problem, then, yes, there is. I have two sets; 30 and 60. The whiffle ball on one side of my 60 set split on the inner side and peeled back on itself, causing a major dynamic imbalance. Left unchecked, this would have wrecked the machine. But I simply dumped the throttle.

IMO, the advantages of using head loaders outweighs the possible problems. They allow you to really go over your ship before flying it, since most of the things that will kill it do not show up unless the engine is running.

Steve