Not true! Photos does retain all previous adjustments (and at full resolution, not like preview files which are lossy JPEGs usually resized smaller). You just cannot change them other than removing them all. (One exception, mentioned below).
I tried this and I do get recovery of details. But I don't think it is correct to say it is "working off the RAW file". It is working off of the processed file from Apertures adjustment stack. It's probably better to think of this as internally a 16 bit TIFF rather than a lossy 8 bit JPEG, but it's not really the RAW. Furthermore it looks like until you enter edit mode (why can't it be modeless like Aperture -- that was one of its best features!) you are viewing the JPEG preview, probably for performance reasons.
By default, yes the previews are of lower quality, but all of the guides on transitioning to LR from Aperture include a step of increasing your previews in Aperture to full resolution / full quality before the migration so that edited versions of your photos can be maintained. The previews have any adjustments you've made baked in, so this would be equivalent to what Photos has done. I also do have the option to edit the original RAW file in LR, which would be he equivalent of reverting to the original in Photos. So from what I can see Photos doesn't have an advantage here.
Regarding my thinking about whether it's a lossy JPEG or TIFF that your working off of, this doesn't really matter much. In both cases you are working off a high quality JPEG (which are still compressed). Even if it were working off a TIFF (which I can almost gurentee Photos isn't doing - it would require a ton of storage), adjustments are still baked into the tiff, so there isn't extra data to pull from for things like adjusting exposure, highlights / shadows, or white balance. It can make these adjustments, it's just that with jpg or tiff it's working with the data it has, which isn't much so the results aren't going to be as good as if you were truly working off of the RAW file.
I still think though that Photos is actually making raw adjustments at least for non migrated files. After I posted last night I was reading over at thephotosexpert.com and they again referenced extensions being able to work in RAW space. If theyre offering this to developers, surely their own built in adjustments are also able to do this.
(Read the comments on this post):
https://thephotosexpert.com/tips/2015/2/5/photos-os-x-beta-os-x-10103-developer-release#.VNeihYY8KnM
I believe this is in reference to the WWDC session referenced here:
https://thephotosexpert.com/tips/2014/6/6/wwdc-session-shows-lens-correction-and-noise-reduction-os-x-mentions-iphoto-and#.VNejcoY8KnM
Watch the video he references. They've changed the way RAW processing is done, essentially something like NIK could work on images in RAW space, creating non destructive edits instead of working off of a baked TIFF like happened previously. I'm guessing that's why the "open in" feature isn't there. That would actually be worse than what they're planning.
This possibility is the only thing that really has me still considering Photos. As an example, I want lens correction, today PTLens can do that, but it works off of a baked TIFF, so today I have to get the image just right before I do the correction, because I can't really make adjustments after correction because I lose lots of data in the process (this even has its problems because images usually lighten due to removal of lens vignetting). With the extensions being talked about here, PTLens could make non-destructive changes to the raw file, leaving me free to make any additional adjustments I need to after lens correction (PTLens has said they'll be implementing the solution in Photos as soon as they can).
So that is where I'm coming from when when I'm asking about how RAW adjustments are made. I think more is going on than what we're seeing, even though the RAW fine tuning adjustments are gone. Those may no longer be necessary because all of the adjustments work in RAW space.
I'm hoping well see a John Siracusa level technical deep dive on how RAW processing is being done in Photos sometime soon.