I repair a lot of Neo Geo boards and I have to say that the 6 slot gets a bum rap for no good reason.
If you change the battery out to keep corrosion away then keep the carts/slots clean AND have a power supply big enough to handle it you will have no more/less problems than you would with any other Neo Geo board.
Don't use some wimpy 7A at 5v power supply. You want the big one. Oh, and one with a fan. Don't use a Peter Chou or Happ switcher that has no fan, or if you run those at their upper ends of power output you should point a fan at them to help them stay cool.
RJ