I've played some very nice hardwood topped instruments. Koa, maple, mahogany, cherry, even birch, and all have had their graces. Oak or ash for example on a small soundboard like an uke would need to be very thin to allow much vibration I would think, and as Dave mentioned would be quite weak.

The headstock angle question has been answered I guess, but I like to reinforce the face and sometimes the back of the headstock with an overlay of hardwood consistent with the woods of the rest of the instrument. With a minimum addition of weight considerably more strength is provided, end grain or runout is protected, and looks are improved. (A mandolin in progress on my bench has a four piece maple neck which won't even need a trussrod, but will get the overlay on front only for looks.) Consider a volute at the headstock angle for more support. Their value is sometimes disputed among builders but it can't hurt and looks cool to my eye.

As for addiction...the disease IS the cure!

Peace and sawdust. Mooh.