Ive never been sucessfully able to dual boot windows and linux on a mobo with secure boot turned on, it seems that is a feature not a bug I'm sure MS would never influence hardware vendors to make it dissadvantage a growing number of linux users.
If you use full disk encryption secure boot is pretty essential, otherwise an attacker can modify the code that asks for your credentials to also log them somewhere easily accessible, circumventing your entire encryption. If you don't do full disk encryption it's still a decent protection against some bootkits.
It can absolutely be more trouble than it's worth. It's not that useful in most desktop computers. But if you are traveling with a laptop it's probably worth some effort to keep secure boot working on that system (and make it more difficult to disable)
Once I am behind the border, I am reinstalling the system with encryption, then proceed to download key material and other important stuff from home over the internet.
I am never letting anyone near my unlocked laptop and if I ever find it turned off e.g. while visiting office toilet, I just assume it has been infected with firmware level rootkit and I am wiping it without decrypting.
If it's removed from my sight during the border check, I assume the same, purchase a new one in a brick-and-mortar shop and sell the infected one when I am back home.
In what threat model? If the attacker has access to your PC they can just as well install a physical keylogger intercepting the signals from the keyboard.
The main use case for disk encryption is preventing data loss when the device is stolen. That's a realistic threat that people face, not boogeyment coming into your house and replacing your bootloader with a malicious one.
I have similar doubts on whether my system is significantly more secure as a result of using it.
the only time it's worth the hassle for we to enable it: travel to the USA, Russia and most of africa (if the country have USA backed airport security, like uganda). pause updates, enable secure boot with a disposable key we don't store anywhere. that on top of the usual FDE with plausible deniability dual boot.
but we still prefer to just fly contributors with blank devices if we can.