I just purchased Pixelmator Pro, and (coming from Aperture), I feel that there is a very annoying thing about using selections with tools: once you have done it, there is no way to see that you have used on what part of the image.
Let me explain:
Say you first do generic color/lightness edits to a photo, then you find you need to lighten a specific part. You select it, then apply the change. Once you have unselected, I didn't find any way to know that there is a global change + a local one, and visualise the selection used for the latter. If you do it for many different additional edits, the only way to go back is to cancel them all (including the global one), which is really frustrating). Or create a layer with each selection and make the edits in each (but "copy and paste in a new layer" doesn't seem to work for RAW images).
In Aperture, you would specifically add a new instance of the tool you want to re-apply, and add a selection (with a brush, but that's not the point) to apply it on the part of the image you need. And at anytime you can visualise the selection mask in color, or unselect the tool, which is very easy and ergonomic (I don't say it should be the same in Pixelmator, it's just an illustration of something working well)
What features would you like to see in Pixelmator Pro?