A real 'U' bend has (or should have!) a smooth flow - one formed from elbows would have severe restrictions and would definitely not work correctly. Even a bottle trap is designed to give some sort of a consistent flow.
Swept elbows might be a bit better, but avoid sharp 90 degree ones.
I've used elbows in a drain system, but only in a general 'downward' direction
I'd advise that if you carry out the work as they 'suggest', get them to sign a disclaimer regarding its subsequent performance.