Rectangle Pro user here -- biggest frustration I've had is configuration. While the GUI is nice for seeing all the options and some basic stuff, when it comes to actually configuring App Layouts it's been a nightmare of clicking around. I went so far as to export my config so I could modify it with a text editor and then re-import it but the config itself includes serialized JSON as values so to accomplish what I wanted to I had to de-serialize the json, make the changes I wanted to, re-serialize it, then re-import it back into Rectangle, all while hoping I didn't mess up the re-serialization step. This ended up taking more time than if I had just painfully clicked through the interface.

Personally, I wish Rectangle had the option of storing configuration in a simple dot file in my home dir but I understand that would prevent syncing through icloud, which is a feature I imagine a lot of folks like. Barring that, I wish rectangle could export/import the configuration in a more editor friendly way so I could make these configuration changes much faster and more easily ensure consistency across settings.