I don't think anyone is going to come smash down your door because you installed Ubuntu. But I don't know what country you are in and two years ago I wouldn't have believed Iranian police would kill people over wearing a hijab. So I think you should do it, but I also think you should stay safe.

I think most chromebooks allow you to disable their boot security? some even allow you to re-enable it with different keys so that you can have a different trust anchor instead of google.

Installing Ubuntu isn't breaking any DRM or any anti-piracy measures.

Unless your country is really strict about using devices exclusively as the manufacturer intended, but that'd be countries that also want to monitor everything you do. Hard to tell without knowing what country that is.

That said, I'm pretty sure Google is perfectly okay with people doing that. Even on the Pixel phones, they openly let you unlock the bootloader, and even allows you to add your own keys so you can relock the bootloader with a custom OS. They only care about security and people not getting a device from eBay full of malware. That's why there's a message during boot that's either orange or yellow warning, to tell the users the device has been tampered with. But everything works fine otherwise.