well not having to set it up is the big one. most good systems i have seen have a baseline map that is specific to the car so its plug and play. alos there is software support from the company and depending on the EMS they have some "bell and whislte" features as i like to call them
like i think its the power fc (may be the greddy) that you canjust plug in a target fuel ratio and the computer does the adjustments
i don't have the perseverance to deal with megasquirt