1st thing arcemu is doing great, they have a great dev team building the core. 2nd private core's are not really private. 3rd private core's usually are stolen form bigger core team like arcemu. 4th where a private core is different then u you will not have much db support, i mean basically no one will support that core. 5th less db support and a hard time money wise = private core's are being shutdown due to lack of support. finally, its a waste of money for private core's they usually suck, unless if they were stolen form a parent core like arcemu, or mongos.
So your pick, core with lacking support or a public core with a lot of support
I trust arcemu and its been good.
btw i just remembered that some private core's are actually blizzard cores that a dev was fired and just made a copy of what he could. So some can get you put in jail.