I'll never do PLC work again. Forget undocumented code, most of the time there's no schematics for the hardware you're working on because it was custom-built thirty years ago.
I hate PLC work for other reasons. I'm starting to look at going back to more traditional software role. I'm a bit tired of the road work and find the compensation for the amount asked of you to be drastically underwhelming. This meme is very much relevant: