It’s not that bad you just kind of end up mashing the pedals in an ever slower way. You end up feeling as if you’re doing 10w for the last minute. Does help to have someone shout at you I can’t see a timer when doing it as it means I pace the effort.
This is an interesting paper looking at the validity of different tests. Comparison of Critical Power and W' Derived From 2 or 3 Maximal Tests - PubMed Conclusion being: shorter the test higher chance of overestimating W’. But as long as you’re consistent it ok.