Are patch plants easier to create than regular agents, since they're a sort of addition to an already created agent (i.e. the garden box)? Or do they basically entail the same amount of coding and work to create?
In my opinion, they're both pretty much equal, though patch plants may be slightly more difficult the first time around since you have to get familiar with Garden Box coding and variables. Beyond that, patch plants and regular plants are generally the same: a coder still needs to write an install script, it is just "nested" inside a GB script. The other scripts, including the timer script and remove script, are basically identical.
So from my experience (mostly helping others--I have yet to finish a patch plant of my own) the Garden Box is definitely not a "shortcut" for people who want easier plants, but those who are new to plant creation aren't in for much if any extra difficulty, and coders who are already familiar with making regular plants will find that patch plants aren't intimidating at all once you start.