i have only been to an international party once, when a friend of mine wanted to meet foreigners for some discussion about world politics (no joke, dude studies politics and philosophy), so i joined and was of course expecting the worst party ever...
in the end it wasnt even the sausage fest everybody expects it to be, i would say it was 40% women and 60% percent men, all between 20 and mid 40s, maybe some dudes have been late 40s...
we even got in touch with 2 girls pretty fast and there was no flirting, or anything like that involved... i might have just been lucky, though i guess thanks to my friend we had good company later...
as for the rest, sure my friend was probably the only person there naive enough to believe that there are people who are not looking for ons/sf/a partner, but to me it didnt seem much different from any other club...
i just tend not to go, because usually im in trouble pretty fast at events like this and i cant stand the music...
im also sure those parties dont have the best reputation, but thats also most of the time foreigners bashing other foreigners, which i never really understood... but apparently it is their japan and not mine, so who am i to say anything about it...
a former coworker explained his disdain for international parties to me years ago, his problem was that because the only thing that makes foreigners special is being foreign, otherwise they all have the same interests, hobbies, jobs and so on, so the more foreigners there are, the less possible it gets to score with japanese women, to which i replied, that i would be very happy to have a foreign friend, who shares remotely the same taste and the same interests and that he should be glad to meet like minded people... looks like some people are happy enough already and prefer to remain on their cloud...
so at the end of the day i also think those parties are not as bad as they think for most people and its maybe also hit and miss...
good advice from solong, though, if you manage to be on one of the horrible ones, just practise and try somewhere else...