I had a MSD404C and a MSD7C unit mounted where the stock battery mounts in my old street and strip Duster, the 404C unit quit at the MATS races after running three years, the 7C was in the car when I sold it 5 yrs later
I've ran the MSD race boxes for a lot of years mounted under the hood, I ruined one by soaking it with water when washing the motor off on one of my old race cars
No other problems that I'm aware of. The 404C unit had broken wire harness in it and MSD said they didn't stock or make those harnesses any more
I mounted my Mopar Orange ECU box on the radiator core support on the passenger side by the horns in my 1966 Dodge Coronet so the harness to the distributor and coil aren't real long