Al-Shafii did not merely disagree with juristic preference, he wrote a treatise against it by name, Ibtal al-Istihsan, refutation of istihsan, arguing that a jurist who sets aside a properly constructed analogy because the result strikes him as harsh or inconvenient has stopped interpreting God's law and started legislating in its place. His objection was procedural rather than about any particular ruling. Istihsan as the Hanafis practiced it had no stated method by which a departure could be checked or reproduced by another jurist, so two scholars invoking it could reach opposite conclusions with no way to say which had actually applied the doctrine correctly, exactly the kind of unconstrained discretion Shafii's legal theory was built to close off.
Hanafi jurists answered by trying to specify the grounds on which a departure was legitimate rather than defending unlimited discretion, naming a conflicting text, a stronger competing analogy, established custom or genuine necessity as the recognized triggers, an attempt to turn Shafii's procedural complaint back on itself by showing istihsan had a method after all. Malik's own school used the device just as freely under different vocabulary, and the reported description of it as nine tenths of legal knowledge suggests Maliki jurists saw less to defend against than Shafii's attack implied, since for them departing from a rigid analogy toward the more equitable outcome was simply what competent jurisprudence looked like.