For me it was working with vendors. Coming from web dev, that was the biggest change, that you can't just open an account with Heroku/AWS/whatever and go about your business like you would if you were hacking for yourself.

You've first got to figure out what kind of SOC you want, and no it's likely not Arduino/RPi or whatever is popular for hacking projects. And no it's not the EE responsibility to choose these things; they just route them. You have to work with vendors to figure out what performance characteristics, lifespan, etc you'll need for your system along with preferences such as OS support (first class, 2nd class, community/experimental, etc, because no, you don't typically just download and install the latest Ubuntu and call it a day; drivers etc are often custom and proprietary, so unless you want to write your own driver stack...). It all doesn't "just work out of the box" like a SAAS service. You've got to put a lot of thought into "other stuff" and make some big decisions before you even write a line of code.