The biggest problem is not so much the ATM machines,
as it is certain establishments along the Riverside that have been known to switch a regular $100 note with a fake one, and calling out the customer on it.
So if you plan to pay for services with a big note like that, do yourself a favor and take a picture of the serial number before handing it over.
But I would suggest exchanging bigger notes to smaller ones before going out on the town.
Some second level expat scammers have tried to switch real $100 notes from an ATM and call the bank out on it, with dire consequences, as all notes are scanned by the bank before it goes into an ATM these days.
I don't know where 'elsewhere' is that you are reading about ATM machines giving out too much money for free by mistake. The ATM's here are the same as in the rest of the world, but some of the visitors using them here, are sometimes pretty lost, and make up strange stories, either from being too drunk or stoned out or just plain poor and dumb.
I've never had any issues with ATMs in Cambodia. As there are many around nowadays, I choose one that looks newish and in good repair. Usually ABA.
I break $100 notes when I spend a larger amount, or go to a money changer otherwise. When I withdraw money from an ATM, I also make sure to withdraw an amount ending in 90, for example $290 instead of $300, as this gives me some small bills right away.