from the cost perspective since you mentioned it, it's not just that it's a whole mirror, you'll find when buying consumers don't want to lose functionality so if their current mirror has compass, clock, reading lamps, on star, auto dimming, manual dimming etc etc they expect the replacement to have the same so from a manufacturing perspective it's no longer a mirror, it's multiple mirrors, keep in mind that little stealth camera you didn't like so much is fine whether it's a Toyota or a Tesla, mirrors become very vehicle model specific, not just in the mount but the mirror side of things as well, that adds a great deal of cost, mirrors also need to comply with a whole bunch of safety regulations that other systems don't have to worry about, again there's added cost, if it's auto dimming that's licensed tech, again added cost
the cost isn't so much a hurdle from a consumer perspective as people will pay for the right solution, even if it is only a smaller slice of the market that is prepared to pay, from the manufacturer/wholesaler perspective it's a lot more inventory which drives up the cost so a larger investment is needed to be in that space, makes a less compelling business case
from a technical perspective most of the mirror designs are stealthy from the cabin side, no so much from the outside, if they are stealthy from outside they generally have poorer levels of adjustment, I've seen lots of different attempts at addressing this, in general from all the models I've looked at I would say if it looked good it was impractical, and if it worked well it didn't look good
getting past things like cost, design, features, approvals, patents etc the biggest annoyances I've seen from mirror designs so far just comes down to day to day stuff, if more than one person drives the vehicle the mirror often gets adjusted so camera aim changes, not a huge deal but can impact performance/field of view, if the person likes loud music this often plays havoc with the image due to vibration, if it has an LCD the mirror quality is often inferior to a regular mirror