I can really only address the first part, and it would probably still be better for you to hear from someone that actually plays falco. Anyways, Falco's autocancel lag is 4 frames if i remember correctly while an L-canceled bair is 10, the obvious advantage is basically what you suggested that there is less lag before the shine comes out... or really before whatever you want to use to follow it up. The main disadvantage is you lose some control over your hitbox since you have to ensure the bair finishes before you land.
Like i said when to use each though would be better answered by a falco main.
