From most studies I've read on the subject, there are a couple big issues. Firstly, physics is hard. It's very difficult to get a useful amount of digital information presented that closely to your eye, where it is in focus, and comfortable to look at. Scaling up resolution beyond a low-res screen is even more difficult. Secondly, tech just isn't there yet. The closest thing I've seen in the market is Google Glass, and that was a huge flop. It also looked dorky as hell, while providing very little real value. Sure, you could read a text, if you squinted and focused your attention up and to the right, but at that point, pulling out your phone is just as easy. It it also makes you look like a strange android while taking your attention away from the real world.
For a product like this to work in the market it has to meet a lot of requirements. Resolution, invisibility (as in, it doesn't feel like you're wearing a clunky awkward device on your face), battery life, safety (you're shining light into your eyes), and provide real, useful, functionality.
Useful functionality, to make it worth caring about a device like this, is good augmented reality integration. And I mean very good. If you put the device on your face and the mapping of the real world stutters for a second, you're going to hate it and never use it - and no one will buy it.
Also, no, being able to project a little cartoon monster on the surface of a table is not "useful" AR functionality...
I would think being able to do that means you've solved most the hard problems you mentioned, so if it can be done well it means we've achieved a certain level of usefulness.
Sort of like how bouncing a white square between two movable white rectangles was a "useful" bit of functionality for consumer AV electronics. Pong isn't exactly blowing anyone's mind now, but it did mean they had to solve a lot of problems to make a machine that could interface with current televisions, deal with user input, do it and update the display within an acceptable time period so it was responsive, and hit a cost and form factor so the general public could make use of it.
I guess the other hard problem, besides just creating the tech, is real-world application,
Reminds me of the Leap Motion device. The creators made some really cool tech, and it works pretty well for what it is, but most people struggle to find a really useful application for it.
Augmented reality is a great dream, but an unobtrusive heads-up-display for simple information would be revolutionary in itself. Baseline applications like a clock/calendar/compass, maybe reminders, or a no-look note-taking tool. Next-gen involving real-life closed captioning, or, when supplemented with a camera and an offline database, a basic "who is this person I am talking to" / protocol officer. Further than that a very rough "am I facing the right way" waypoint finder, etc.
If the hardware can be made cheaply for this sort of application, use cases will emerge faster than you can shake a stick at. Overlaying reality, etc., are way less interesting without huge amounts of compute.
Galvo projectors as used in hololens etc. are vector, but tiny projectors aimed at retina are extremely hard to pull off, even ones aimed at glasses are very hard and lousy. Aiming them at a wall is ok though.
I do know that the mems, fiber coupling to the mems and the waveguides necessary for wide enough FOV and variable focal length are very much bespoke right now and thus costly.
At higher scanning rates the achievable angle of deflection with commercially available MEMS scanners (the type of scanner used in the 720p pico projectors) is too small to be useful.
The other class of laser scanner potentially relevant to practical high resolution projection are the solid state laser scanning systems, which suffer from limited angle of deflection AND a limited number of resolvable angles.
60fps 8k is achievable with a mechanical system using a turbine-driven polygonal mirror. Unfortunately, the precision machining, several tens of kilowatts input power and requisite hearing protection present some obstacles to commercialization.
As a general rule, if a raster laser projector claims to achieve much over 720p they are either lying, mistaken, or out of your price range. It requires an order of magnitude increase in scanning rate to scale from 720p to 8k. The next incremental milestone for this sector is "actually achieving 1080p instead of just lying about it".