The downside to fully assembling is your brush can't reach into all the crevasses you might want to have painted. You might view that either as a place where there should be less detail because it is shaded, or as an unacceptable loss of painting detail.
With Hellions, I would think you should be able to reach all the relevant parts, and the arms on the riders would be a bigger issue than the riders and the boards. Reavers would limit your ability to paint the bikes or the riders' chests properly, so might be sensible to do separately.