The fast rate of growth/change for Gravit can be a pro in that in means plenty of new features. However, it also makes the product fairly unpredictable. For example: In the past year, Gravit was open sourced, then closed sourced again.
You can rescale assets without quality loss, easily export x2 assets for retina designs and design high quality icons & artwork. Being pixel aware lets you set a grid and snap objects to it as well as round to the nearest pixel edge to clean up your layers.
Froont is a web app that runs in your browser - not a full desktop application. While the developer touts this as a 'feature', web apps cannot be as full-featured - nor as stable - as desktop apps (specially when dealing with projects that might be large, or complex).
Webflow has four fixed breakpoints (desktop, laptop, mobile landscape & mobile portrait). The inability to set your own media queries can be limiting. If your layout breaks outside of these four options, you have to export the code and write them yourself.
Pidoco has a focus on collaboration and provides plenty of features to make this easier to do. Pidoco is browser-based, making it easy to share designs with clients and colleagues. You have control over who can edit, and who can only view the design, and you can collaborate with others real-time.
Features such as 'Layer Sharing' allows you to quickly update shared components such as headers across multiple pages. Other mockup tools such as Mockingbird will require you to manually propagate changes across all the pages.