So, what exactly can Pro do that Express can't?
Quite a bit.
Express activates on 1 computer, Pro on 3.
Express has an absolute maximum vertical Timeline resolution of 2160 px and a maximum horizontal Timeline resolution of 3840 - so 4k UHD. Pro, with a powerful enough GPU supports 8k and higher resolutions.
Express has a maximum color depth of 16 bits/pixel. Pro supports 32 bits/pixel.
Express cannot support third party plug-ins, besides the available Mocha and Boris 3D Objects. Pro can use hundreds of third party, OFX plug ins, including sets from Red Giant, NewBlue, Re: Vision, Boris/Imagineer (including Mocha Pro), and others.
Pro has at least 80 exclusive effects and other features, like Foundry Camera Tracker, Surface Studio, Exposure Pro, the Depth Matte and Depth Map functions, the entire Channels Effects group (6 effects), most of the Behaviors Effects group (50 effects), the entire Animation effects group (5 Effects), Dehaze, Highpass Sharpen, Remove Stock Background, Inner Glow, Outer Glow, Distance Field, Tone Coloring, Motion Lock, Chromatic Aberration, the Motion Track EFFECT (which is way easier to use than the 1 and 2-point trackers in Express), Custom Light Flares, Auto Stabilizer, and support for import/export of additional formats, including EXR.
Express uses the encoders built into your OS. Pro uses third party libraries with a wider range of settings.
This list grows with every release. I'm one of the Beta testers and the next major release is going to add even more Pro-Exclusive effects and features. Don't ask for details about those. My NDA means my lips are sealed.
Here is the splash page from FXhome on what you get and don't get.
@Andy001z I need to update that PDF I used to maintain comparing Express and Pro. I haven't updated it since HF9, but, it was easier to read than the FXhome page - at least on a phone. Side by side columns rather than vertically stacked columns for ease of reading.
Yes definitely use a desktop computer to cute the page, even in desktop mode on phone it still hard to compare.