What about adjustments made via brushes? Are these retained? You might not be able (yet) to make such adjustments in Photos but it would be interesting to know if Photos has the ability to read those adjustments.
Any adjustment tool. Again, image edit data is stored as the numeric
results of the edit, not as the settings of a particular tool. To greatly, greatly over-simplify, if an edit changes the brightness of a particular pixel by +2, then the result of the edit would be saved as the equivalent of, "(pixel address) +2." It wouldn't matter what tool was used to make that change, the change in the end comes down to a specific number that would be added to or subtracted from one or more of the original pixel's parameters (a color adjustment might be +23 Red, -109 Blue, 0 Green).
Clearly, storing edit data on a pixel-by-pixel basis would be highly inefficient. So a Highlights adjustment may generate an instruction equivalent to, "Adjust the value of all pixels brighter than X by -Y." The same adjustment made with a brush would include a "mask" - a description of the boundaries of the area to be edited, like a series of map coordinates. It might look like this, "Adjust the value of all pixels within the area (boundary list) that are brighter than X by -Y."
So long as the Photos render engine (the program that generates the final image) interprets those instructions in the same way as the render engines in iPhoto and Aperture (and it does), then you will get the same results.