Not necessarily true. Sometimes new prescriptions can take some time to adjust to if it's been a long time since the last prescription. Once you settle in they are 100% rock solid but it can be a few days of weirdness as your eyes adjust.

The biggest thing is that your eyes will naturally "strengthen" to make up for loss of acuity with an old prescription and a new prescription will correct that immediately and thus you end up with a mismatch that takes awhile to settle.