"FeCr was always intended to be the bridge platform between Type II and Type IV and unfortunately suffered from delamination issues as well as equalization/compatibility problems. That said, FeCr Type III's experienced a longer useful run than pure chrome ever did - in the consumer space. Lastly, what the FeCr formulation brought to the table was a significant blend of Type I low frequency response coupled with the sharp response of Type II's - FeCr was more akin to what good metals ultimately offered in terms of extended low frequency response."
BASF never experienced delamination issues with FeCr because the base ferric layer was not cross-linked, for better adhesion to the PET film while the upper layer of chrome did use a cross-linked binder for lower rub-off. The same applied to double-coated chrome tapes. I don't know enough about other ferric-chrome tapes simply because their significant inherent limitations made them less interesting than advanced ferrics, chromes, and metal formulations. Whatever benefit in low frequency response they offered, it was simply due to thicker coatings that resulted in more magnetic material that increased MOL. Since the bias point was higher, that decreased LF distortion more, allowing even higher MOL values than those provided by a thicker coating. However, AH10 (SOL at 10 kHz) values were worse than ordinary ferric tapes because of coercivity mismatches between the two layers. A cobalt-doped ferric base layer would have been one solution, but BASF and TDK were already double-coating Type II tapes with the kinds of results they wanted. I'm not sure what you mean by "extended low frequency response." Extensions at the low end are a function of head geometry, not the dispersion.
BASF and all other ferric-chrome suppliers dropped Type III tapes by 1982-83. BASF continued to coat pure chrome tape until 1996 when it sold the magnetics division to Kohap as "Emtec," and Emtec continued coating chrome until it failed in about 2003. Pure chrome tape was a large part of the European retail cassette market, and in the U.S. BASF pure chrome tape was in the fifth position in the Type II market until 1996 when the consumer division was killed off. In the music duplication market, pure chrome and the subsequent 80/20 chrome/cobalt "Chrome Extra" was 15-20% of all the top Billboard releases until cassettes faded away. (TDK tried to crack the duplication market in 1987, but pancakes of TDK SA were too dirty, too inconsistent, and too badly wound to replace BASF chrome tape.) So, if ferric-chrome died in 1983 and pure chrome lasted until about 2003, how is that considered a "longer useful run"?