The 400 is likely better for tactical airstrips. Although the C-17 touts rough strip capabilty, it is mostly used as a hub and spoke type transport.
True. Mostly. In
current USAF service.
But during the entirety of the Iraq and Afghan wars, C-17s flew from major air depots in Europe and Turkey directly to very austere forward airfields in both Iraq and Afghanistan, which USAF calles "direct delivery" and at which the C-17 excels. And C-17 is flying into plenty of very austere fields deep in Africa right now. The C-17 cannot hope to match the A400's ground flotation (reportedly 100 passes on a CBR 6 field?), but there are plenty of 3000 ft CBR 9/10 fields all over the world.