You need to use global variables, not use heap allocations, avoid stl library classes, avoid strings, etc. The constraints due to the minimal resources mean you need to throw out elegant and robust programming in order to perform well. So the code ends up looking super hacky.
That said, I'm not sure what I'd consider to be hacky code in a constrained environment. Looking at https://www.reddit.com/r/learnprogramming/comments/3cd0x4/wh..., for example, many things people feel are hacky don't apply to Arduino programming mainly because there is hardly anything external to interface with.
PLCs until recently only had global variables and a great deal has been accomplished with them.
A lot of machine controls and embedded work is more waterfally and less agile. Figure out what it has to do, make it do that, then it is done and you hand the machine or plant over to the customer and you can’t change it.