How to airbrush an even coat on a complex undercarriage??

I would say shoot the model from all angles, but don't let the paint build up much. That should cover it fairly well. Unless you plan to display your model as an overturned vehicle you won't see much of the under carriage anyway.

Use thin, thin paint and resolve to have to make up to five to seven passes from various angles, Painting that kind of affair just becomes a separate "job" to do, like road wheels.

Plenty light coats at low pressure - I'll generally give the inner / hard to get at areas a few coats before moving onto a general coat. I find flat acrylics excellent for work like this as the quick drying time allows you to coat / dry quickly & build colour.

i do a lot of warwheels and trucks and RR stuff so i have this issue every build. unless you roll it in a ditch most of it won't be seen but we're modelers so.....

usually if it is an open frame you can prime and paint from top and bottom and be ok. maybe do the differentials and engines separately. if it is a solid hull like a modern 8x8, get as much on as possible attached primed and painted before other stuff like differentials and drive shafts. prime and paint them separately, attach and a final base color coat. and, if all else fails, MUD is your friend.