I'd say the form factor. The raspberry pi is too thick for a tablet.
But it's not scalable, and I'm not particularly good at soldering. ;-)
The compute model is the right choice for an actual product (i.e. making more than one), but for someone trying to jam a pi into a thin space, removing the ports is almost as thin. Plus you don't also need a motherboard.
The compute module is also still waiting for it's BCM2711 refresh, another reason it's not the same thing. But when that does arrive, if it's just a case of swapping the modules that'll be a brilliant upgrade for this project.