Why its bad :-
Awkward / un ergonomic hand postion - When your left hand is at its usual position over the keyboard, you would have to lift up your wrist in midair while your thumb reaches out for the CTRL key so that your ring or ittle finger is able to press the tab key simultaneously (or you could rotate your hand in the other direction so that your little finger is at CTRL and your ring finger is at tab, but this puts a little strain) either way, pressing ctrl + tab is just uncomfortable.
Having to press 2 keys takes more time - This one would only mildly annoy me since there are many many functions which require you to press 2 keys, however those functions are usually not amplified by my first point.
Both of my above points defeat the sole purpose of pie menus which is speed and quickness, i would usually settle for having just one of the above but both of those two problems in relation to each other just make it slow and uncomfortable to use pie menus, even if the shortcut was changed to something like SPACE + TAB the second problem would still exist, in my opinion, in order to have the most efficient pie menu incorporated both of the problems should be eradicated (and the first one only exists in this special case)
Possible Solution (includes video) :-
Before the alpha version of 2.8 there was a novel solution to have both an option to directly access edit mode and the pie menus with just the single TAB press! However since it required the detection of mouse movement after the keypress it made directly accessing the edit mode sluggish because only after blender confirmed there to be no mouse movement (which took a couple hundred milliseconds) did it take you to edit mode.
Now as i demonstrate in my video, Changing the double click setting in the user preferences gets rid of that sluggishness, Have a look.