Are there many differences between Ultimaker Cura and FlashForge FlashPrint slicers?
The commonality of the 2 slicers is that both are developed and maintained by a printer manufacturer. The largest difference is that FlashPrint is closed/proprietary software, while Ultimaker Cura is released in source (so-called open source project) to the public; this is valid for both the frontend (Cura) (Graphical User Interface) as for the slicing core (CuraEngine). Basically this implies that there is a larger community developing and bug fixing the software. Also, FlashPrint is exclusively available for the FlashForge printers while Ultimaker Cura can be used for different brands as well.

Statement from www.3dprms.com:


  The Flashpoint software is an in-house software program developed by FlashForge for use exclusively with the FlashForge 3D Printers


Statement from the Cura wikipedia:


  Cura is an open source 3D printer slicing application. It was created by David Braam who was later employed by Ultimaker, a 3D printer manufacturing company, to maintain the software.


As FlashPrint is proprietary, it has no shared source repository and can therefore not be based on existing forks of software that are released under e.g. some version of the LGPL license as this implies that you need to share the amendments you made to the software, otherwise you would be in violation:


  ...any developer who modifies an LGPL-covered component is required to make their modified version available under the same LGPL license..




Note that discussing the exact differences in features between the 2 software packages (e.g. implementation differences of model support structures) would be more fit in a forum style discussion board rather than on a Stack Exchange site.