There's also the big difficulty to harvest laser beans...
But seriously, I wouldn't say this one is less efficient, both attacks are quite different in scope.
The laser-based attack requires line-of-sight to the device and apparently works only on stationary home-assistants (i.e. your Google Home), while this ultrasonic method explicitly targets Smartphones and has potential for wide unfocused attacks at public spaces (i.e. by rigging a table in a coffee-shop).
But seriously, I wouldn't say this one is less efficient, both attacks are quite different in scope. The laser-based attack requires line-of-sight to the device and apparently works only on stationary home-assistants (i.e. your Google Home), while this ultrasonic method explicitly targets Smartphones and has potential for wide unfocused attacks at public spaces (i.e. by rigging a table in a coffee-shop).