It is surprisingly not.
I travel frequently with random ESP32 projects in various stages of looking highly suspicious and never get hassled about them. 4oz of an unfinished bottle of water though is absolutely not getting through security.
Do those have large li-ion batteries? Lets say you'd diy your own charge controller, I could see that get treated differently since you could cause a nasty fire that way.
Not large, but they do often have some kind of battery.
In the US, the TSA and FAA have limitations on how big of batteries you can bring - and you must carry them on.
A nice 3D printed case and some fake branding would probably minimize any possible hassle.
In London security will sometimes specifically ask if you or anyone else has made repairs or modifications to any electronics. Depending on how honestly you answer, this could still be a hassle.
The secret is to travel with such a large quantity of homemade electronics that you get classified as "harmless nerd".