I cannot speak for everyone, of course, but I get good code for my Haas mill.
I do have about 4 different variations of my Haas post, with slight variations between them for certain types of work I do with different machine setups. I can easily create these custom posts myself within the OneCNC GUI.
OneCNC uses a common interface within the software for setting up the post, so users can make post adjustments in simple dialogs.
To date, with OneCNCXR3 being the current version, the post system does not automatically handle subs or macros. So I don't want to pass on the impression that we do weird and wonderful things with our posts, but we get good, solid code out of it. I'm thinking that our relatively simple post processor settings are there to help ensure crash-free code for all users, and relatively few support issues caused by the complex code manipulation possible in some other cam systems.
Contact OneCNC sales for specific questions that would relate to your specific questions on your machines and general machining requirements. I think they can demo the post setup for you online, so you can get a better idea of what it is about.