Not only is he almost always injured having only played one full season in his five year stint in the NFL, but he lacks even the average coverage abilities that one woud expect to see from one of the best safeties in the NFL.
Indy fans will usually justify him not getting as many tackles as he should by saying that people run away from him. Although this is true, there are plenty of hard hitting safeties that have managed to put together 100+ tackle seasons regardless of offensive players running the other way (Rodney Harrison, Gibril Wilson, Adriwn Wilson are just few of many).
Also, any grown man who runs under a 4.4 40 yard dash time and has no problem running full speed at the line of scrimage to get tackles that your front seven failed to get because they are too busy getting their asses handed to them by offensive linemen can play Safety for the Colts. As Proof they drafted a Free Safety in the sixth round and he gets numbers almost identical to Sanders who is the strong safety.