Everyone's pretty much said the rest, so I will just add a couple bits.
Mopeds, whatever country they are made, are made with local parts in mind.
And what causes some of the difficulty, my best guess - is the difference in heat range even on plugs that are identical on paper.
NGK kick ass in Yamaha, Honda, etc.
Bosch kick ass in Puch, Tomos, etc.
Apparently Japanese and European heat ranges don't seem to match up exactly, and if you are running a "Tricksy Mix", trying to squeeze a hair more performance....yeah, yer likely to run into problems.
But thing is, mopeds, generally, seem to be as individual as their owners...I've seen matched pairs have different "preferences" as far as mix, plugs, etc.
For whatever reason, mix, carb adjustment..whatever it may be, not once have I gotten any decent performance from an NGK Plug in a Tomos, but remember, alla my 'peds are individually tuned, by me - YOUR milage may vary, as it were, although I've seen enough Tomos owners cussing over the same problem in my time.
My current problem was a combination of dying plug, and dying coil, and I've not seen that happen before, not both at once...so imma check for an underlying problem that might have caused that.
As fer splitfire, I've had a couple of folks I knew on the east coast bitch to me about knocking a hole in their piston with them, apparently the spark hits hard and concentrated right on the top of the piston, and with consistent hard use, it more or less drilled a hole in it.
I didn't believe the guys till they brought a piston to show me, and yeah, there was a hole right through the top.
HOW that occured goes beyond my knowledge of engines, but best guess would be it runs way too hot and concentrates that in a small area, and moped pistons are much smaller than auto or cycle pistons.
Of course, they were running a thinner gasket than reccommended too, and that might have had to do with it.
General reccommendation is to use the plug the manual calls for, and if that's not possible, at least try to stay with plugs from the same continent.