Any single source of paleoclimate information has its limitations. Use of tree rings does not apply in polar regions where trees don't grow, and tropical trees do not provide accurate information. Also, trees don't reveal conditions outside the growing season.
Analysis of isotopes of oxygen from corals is limited to tropical regions and do not give multi-century information. Ice cores provide data from limited regions of the globe, places trees don't grow, and as such are complementary to tree rings and corals. However, it is difficult to resolve individual years in ice cores. Mann (2002) describes these techniques in more detail and encourages use of "multi-proxy" methods to avoid inadequacies of single methods.
Reference
Mann, M E., 2002: The value of multiple proxies. Science, 297, 1481-1482.