I build out of redwood fence boards all the time. It is light and unlikely to warp. It is brittle and will split. I do not use finger joints, but lap joints reinforced with cabinet biscuits. It's not going to take forklift and weight abuse, or much winging around. However, the boards have high R value (light, porous, dry). The fenceboards are usually a loss-leader deal, you can build mediums (at current prices) for 2.50. Fenceboards are 11/16th, so dimensions are slightly undersized. Make sure the top bar rabbet works.
There are other threads about reusing old mill-work. My perspective: it is more valuable as a resale item -- sell it and buy new material. If old mill-work has been painted, you must assume the paint is lead based.
I have built plenty of redwood hives. I am building twenty supers with dovetails out of a log I milled up about four months ago. The wild bees here live mostly in old barns and houses made of redwood. I have never seen a hive in a pine tree here, i think they must like the redwood.
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