CONs
Skymiles has a lot of partnerships, but you can't really enjoy several of them if you are based outside US/North America.
Flying 'discounted' - discounted??? - economy with AFKL and some other skyteam partner can be a problem: no Medallion Qualification miles or segments on some classes.
DL requires more miles and more segments for elite qualification.
PROs
https://www.delta.com/skymiles/about...ance/index.jsp