You are considering only the extremes. There's also a ground between ordinary adversaries and nation-state agencies.

Take for example China. There are plenty of rumours about business executives taking throw-away electronics while crossing the border or how China installs some random software on people's devices when crossing a border neighbouring a certain province[1]

FDE can also lead to increased suspicion when crossing a border and refusal to unlock the system can pretty much lead to a denial of entry or, worse, detention. FDE also doesn't help in cases when a random border crossing can require installing a malicious boot-loader or a persistent malware somewhere in the system.

In the China story, they might not be installing a persistent bootloader but there's nothing really stopping them from doing that.

Honestly, in certain cases (and I'd rather say, a lot of cases) it's just easier to have no device than deal with what they did to your device after the fact or just sell the device after entering if you are suspicious of it.

[1] https://motherboard.vice.com/en_us/article/ne94dg/jingwang-a...