If shooting a VERY fast arrow, even the most determined tuner may still have problems tuning a large diameter fixed head (assuming they want a large diameter...were talking 300 fps-ish and 1 1/4" head or so).
Also when windy the mech heads DO have an advantage.
Modern mech heads kill deer as dead as anything...and usually just as cleanly.
Personally, I have had good luck with mech heads but am still shooting fixed head as the mech heads are frowned upon buy some of the " old guys" . when in Rome...