IMHO, the only thing they could screw up badly would be the transducers and until you know the boat. knowing the best spot to put them is a crap shoot to some degree.
I would have them install nylon boards on the transom to mount the transducers , then there are only a few holes in your boat and you can add subtract or move the transducers and what not without any problems... and you get them installed for free....
the networking wire, as long as they can be accessed they are easy to move, if are hooked up wrong...and who knows maybe they have installed lots of them and know your boat so will do it right the first time
but that's just me