It's more complicated than that. Yes Ren'py is designed to work with visual novels and such, but ultimately it's a Python based software that is simply tied together to make things easier for that purpose. You still have to code in all of the quests triggers (Where, when, how the quest is triggered), what kind of variable are used, where and when characters start to speak, what poses are used, super-impositions, etc...
Then you have a bunch of things like mini-games, quest trackers/UI elements, inventories, etc...
The ex-coder Dogeek was known for basically being a douche and wrote overly complex and poorly optimized code, with poor naming conventions and a lack of comments on said coding. Which is a massively unprofessional and inefficient.
Say i present you a wall with a bunch of light switches. A good coder will properly and clearly label each switch and their purposes, and will be neatly arranged on the wall (This one turns on and off the lights in the kitchen. This one activate the oven. The other one activate the automated turrets, etc...) Most, if not all the switches should work as intended. If one doesn't work, fixing it should be fairly straightforward
From what people have told, Dogeek didn't bother naming any of the switches, are all bundled up in a rat nest on the floor and you have to figure out which does which by error and trial, and cross fingers you don't accidentally burn up the whole place. Since nothing is explained, you don't know that the original coder made one switch work by tying two specific wires together, etc...
Now imagine that the quantity of switches is in the thousands (tens of thousands even).
