Does anyone know of any reason not to run a Surefly with a firewall mounted EarthX?

Im not sure the EarthX is approved for firewall mounting on a 180, pretty sure the stc put it in the standard aft battery box.

That is correct. However, if one has an existing firewall mounted battery, he/she can ask their mechanic's opinion as to whether or not the STC's are compatible and proceed from there. I have not had any issues with my surefly and EarthX firewall mounted battery. I installed a push/pull circuit breaker instead of the supplied fuse breaker as I operate on floats and I didn't find it easy replacing the fuse breaker in the event that was required.

I had that on my cub, but after 5 years, during a trip on skis the fault indicator light came on, and i was nervous to fly it back, i forget the voltage but the earth x will shut off to save its self. Sitting ontop of a hot earthx with the potential to shut off, and kill the mag, i opted to put the oddysey back in. The earthX cranks better for sure, but the sbs J-16 will run the mag until the battery dies totally

Once upon a time, the STC for the Surefly required a battery capacity (20 AH?) that was higher than the EarthX options (~15 AH). That may have changed now that Surefly has battery backup solutions, but it might be worth checking.