Backer Boxes & Backer "Channel"

Good morning/day/eve - A couple of general questions on backer boxes: 1. Is it reasonable (or profoundly unwise) to consider building a single "backer channel" to house a row or recessed lights and speakers in a soffit, rather than individual boxes for each light and speaker? 2. What about a very large backer boxes, such as to hold a large ceiling cassette HVAC unit? (The unit I am considering requires an 18"x48" ceiling opening). I am considering this as a visually more aesthetic solution to the more common (in music studios, at least) but uglier wall-mount Pez-dispenser style. Thank you for any advice, experience, humorous anecdotes. Best regards