Yes, I would love to be able to select multiple objects, get “Mixed” in the width/height fields but then when I enter “+20” into the field (or “Mixed+20” I guess would also work) every object would be increased by 20px…
This is one area where Sketch still maintains an advantage over Figma. You can perform some nutty operations on layers in Sketch just with a few operations in the x, y, width or height field. You could resize multiple layers of different sizes by the same amount, sure; but what you if you want to scale a layer to 50% of its current height, minus 100px, while still keeping its bottom edge at the same position? Sketch can do all that with a single input.
(After playing around a bit I should acknowledge that you can combine operations in Figma: 50%-100 will do what you think it should, even with multiple frames selected. Strangely the simpler -100 won’t. You have to specify 100%-100 and that will work on multiple. You can also use 100% as a base to shift multiple frames by same amount in the x or y.)