Honestly, I think this is still fine for most people. The probability of a border agent asking me to unlock my phone is very, very low. The inconvenience of using a burner phone is high.

If they do take my phone (completely shut down, unlikely they'll be able to break in) and it's gone forever, that sucks, but then I get a new phone, restore from a backup, and move on with my life. Given that the probability of getting to this point is very low, I'm comfortable with the risk.

But sure, if I was at high risk of being detained at the border due to my profession, country of origin, ethnicity, etc., I'd probably look at this differently.