On Mac by default the key repeat time is not enough sometimes to get in bootrom mode. Normally I use a coffeecup and put it on the spacebar while I go to the switch and power it off and on.
But on Mac you need to set the key repeat mode on the fastest setting to make this work.
You can set the mac keyboard repeat to be as quick as windows with a shell command:
defaults write NSGlobalDomain KeyRepeat -int 0 (which is quicker than you can set it in keyboard settings). You need to logout and login again before this is active.