The lifetime (in displacements per atom) of RAFM steel still isn't that great. Moreover, after some irradiation, the material becomes brittle unless hot. The temperature window (300 to 550 C) between where the material becomes sufficiently ductile, and the upper temperature where the material undergoes creep, is rather small. The upper temperature isn't that high, either, which makes high temperature blanket concepts harder to design.

RAFM steel is also ferromagnetic, which interferes with the design of the reactor.

You can find a lot of info on material selection in this PDF:

https://gcep.stanford.edu/pdfs/qa4ScQIicx-kve2pX9D7Yg/baluc_...