I use Sparkle for a week now and I am very pleased!
But the two things I really, really miss are:
More devices and preferable a way to create a device myself. The sizes that I really need for mobile development (and to cover the majority of the mobile devices) are a screen width of 360, 375 and 410. The 410 is suitable for the latest iphones (414) and the samsung ones (411, 412).
When making multiple layouts (for each device) it would be very handy to click an element (text box, image, etc) and to have a button ‘Adjust to screen width’. Before clicking such a button you can set a margin for how many pixels it should keep distance from the sides. This would substantially speed up the design proces!
EDIT: For reference (screen width of almost all mobile devices). Scroll to the bottom. https://www.mydevice.io/