The difficulty with a “warning” is that each situation can be incredibly different based not only on what someone has as a pre-existing system, but also what they plan to install and how they want to configure and operate it (storage or not, etc.).
It’d be really hard to give warnings to cover all complexities, without it turning into something that boils down to “don’t try this at home”—when in fact nearly of this is learnable and do-able, in one way or another, by anyone who’s got some real DIY aptitude and drive.
The same thing that leads to the wide variety of complexities and variables in hydronic systems also means that there are a wide range of possible solutions, which’ll also vary a lot with field conditions, budget, and personal preferences.
If I knew what I know now, I’d have definitely done some parts of my install differently, but that’s not to say I have regrets, because nothing has turned out badly and I would not have learned if I hadn’t taken the plunge at some point.