When comparing Final Cut Pro X vs Flowblade, the Slant community recommends Flowblade for most people. In the question“What is the best video editing software?” Flowblade is ranked 6th while Final Cut Pro X is ranked 12th.
Specs
Ranked in these QuestionsQuestion Ranking
Pros

Pro GPU utilization

Pro Powerful media organization

Pro Resolution independence

Pro Real-time graphics and effects

Pro 64-bit architecture

Pro Native format support

Pro Full high-quality pro-res support
Pro Integration with Motion

Pro Multicam editing

Pro ColorSync-managed color pipeline
Pro Powerful and light-weight
Pro Multifunctionality
Pro Free and open source
Flowblade is licensed under GPLv3 with source code available on GitHub.
Pro Stable
Has an interface similar to OpenShot and PiTiVi but doesn't crash when previewing videos or after few render.
Cons
Con You need lots of plugins
For example tracking is something Final Cut can't do. Only with a plugin which costs (example) around 100$.
As well as for advanced colour correction you have get a third-party plugin.
Con Important compatibility and format issues
Breaks compatibility with previous versions of FCP and does not support industry standards such as EDL, OMF, AAF. You can export to their version of XML and use third-party tools for a workaround (with limited success).
Con Works only on OS X
Con Color grading is unfamiliar and nonprofessional
Especially compared to Premiere's Lumetri Color Panel in 2015 cc.
Con Linux-only small user base
If you have a friend/machine running anything other than Linux, it won't work. So you cannot transfer knowledge as easily and this also results in a rather small user base so also fewer tutorials and online support.
Con Lack of advanced editing features
Doesn't support stabilization or complex transitions for example.
Con Requires higher display resolution
Con GPU rendering is unstable
A fair amount of hardware and driver combinations crash the software.
