IMO the 24-70 is too short a focal length for portrait work.
That depends... on an APS-C camrea it's a particularly nice range. Near the 70mm end of the range is fine on FF too. It's only slightly less than the "ideal" portrait lengths on FF: 85mm to 135mm. (50mm to 85mm on APS-C.)
Andre is right, though, there will be some wide angle distortion using the lens at the wide end... but in general the 24-70 is one of the best corrected, least distorting of all zooms.
Regarding softness... Did you buy it used? If so, they do get wear in the mechanisms over time, that make for some erratic focusing. Or, if it's new, it would be unusual but perhaps it's not be properly callibrated.
An article at Lensrentals.com discusses their tests of 100+ copies of the 24-70 in their rental stock, well used I'm sure... and explores what seems to go wrong with the lens. Note in particular the chart showing test results before and after a tune up by Canon techs (nine copies of the lens).
This isn't a damnation of the 24-70. This sort of variation isn't all that unusual, though it's rarely seen in brand new lenses of high quality (such as Canon L's). It just so happens that Lensrentals has a lot of copies of the 24-70 to compare and test, and that it illustrates their points in that article well.