Melonite can be, and is, applied to 4140 stainless steel, a steel commonly used in the firearms industry for the making of barrels and slides.
On a carbon steel slide, such as used by Glock, the nitrocarburizing process is a quench/polish/quench (QPQ) in the molten salt baths that treat the steel. On a carbon steel firearm part, all three steps would be needed to impart the best possible corrosion resistance.
A 4140 stainless slide starts with an inherent resistance to corrosion by being composed of ~12% chromium. The stainless steel upon oil quenching and proper heat treating, will also have a higher initial Rockwell hardness number than its counterpart made of carbon steel.
Why this matters is that a stainless steel slide will only require the first quench in a nitrocarburizing process and not the second, that would be absolutely required for a carbon steel matrix to become protected from corrosion. A stainless steel slide or barrel undergoing the nitrocarburizing process will undergo a single quench and polish. For a stainless slide or barrel, one is not doing the QP process solely with an eye towards corrosion resistance, because in part this detail was addressed by the initial material selection, but with the concern being a further case hardening the part as well as providing a highly wear resistant blackened finish. Making the slide as hard as is practical without becoming brittle is a valid objective of the process, because less hard metals such as keys, flashlights, whatever, that come into accidental contact with the slide will not likely harm the finish.
So, it is not the case that stainless steels cannot be Tennifered or Melonited with any success, it is that they do not require the entire three step process that a carbon steel slide does. The second quench is not only undesirable from a brittleness standpoint, it'd be pointless from a corrosion proofing standpoint as well.
