I have to agree, especially in war zones tab targeting is very weird and I usually have to resort to trying to click on my target which is obviously very difficult from range. Besides the already stated problems of tabbing to targets at the outer edges of the field of view, it also seems to like tabbing to targets well out of range, like 3x or 4x farther than a casters maximum cast range. WoW always had a tab target range console command but the default was only slightly longer distance that standard cast range.
It is also somewhat annoying to not be able to tab to targets standing just slightly out of line of sight but still visible, for example, someone standing behind a box or pole or something will not be an option for tab targeting.