There's a sort of vicious circle attached to underrated conductors. By the very fact of what they are, they usually conduct third string orchestras or, if they did occasionally conduct a first class one, probably no record remains, or they recorded only as accompanists. Appreciation of Swarowsky, Lindenburg and Ackermann (apart from the Philharmonia operetta recordings) is hindered by the conditions under which we are obliged to hear them, or by the incompleteness of the repertoire they set down. Add Perlea to this list.
A signal case is Vladimir Delman, whose work we virtually know only through off-air recordings with Italian radio orchestras, three of which (including his own in Milan) were teetering towards extinction. Listening through these obstacles, we can hear that his Tchaikovsky, Shostakovich, Berlioz and probably much else were up there with the best, but not everybody wants the bother of doing so.
Another question might be WHY some potentially great conductors never got beyond conducting third-rate orchestras. Just bad luck? Evil machinations by other conductors? In the case of Perlea and Delman, certain characterial issues seem to have made them their own worst enemies.
Lastly, Anatole Fistoulari's one recording with the Concertgebouw sounds like a great conductor at work, but most of his recordings with the London orchestras are just good-to-excellent. Underrated even so.