What are some alternatives?
When comparing Pencil2D and Synfig Studio, you can also consider the following products
OpenToonz - OpenToonz is an animation program that is a free software version of Toonz, the animation software...
Adobe Animate - Adobe Animate is a Flash, vector animation software.
Pixelmator - Pixelmator is an image-editing application for Mac and iPad.
Spine - Spine is 2D animation software designed specifically for software and game development.
Xara Designer Pro - Discover the only software you need for all your creative work: Powerful illustration and photo editing tools, flexible page layout and unrivalled WYSIWYG web design.
Pencil - Pencil is an animation/drawing software for Mac OS X, Windows, and Linux.