I did a 0.8mm fanout on their older limits, and it was barely doable - the inner balls of the ECP5 FPGAs are all power and ground, and with careful depopulating

I did a 0.8mm fanout on their older limits, and it was barely doable - the inner balls of the ECP5 FPGAs are all power and ground, and with careful depopulating of the inner layers' annular rings it's possible to get enough working room to make it all fit! But if you want signals in those inner balls... still pretty tight. And the 15x15 ECP5 is the absolute max this would work for; after that you hit both needing inner layers to route on, and additional ground planes for the outer layers' return current.