It can be used... pretty much any time! Just throw it out, see what happens. JUST DO IT. Oh, it whiffed? Who cares lol, they can't whiff punish it on reaction. Oh it got blocked? Perfect, you're at -3! That's the ideal time to... DO IT AGAIN. iMCF recovers in crouch, so at -3, your i9 iMCF comes out at i12. That's faster than a standing 2A, and anything faster than that is a high... which will be TC'd. That's right folks, iMCF frame traps into itself.
Against characters who don't even have a fast 2A, -4 to -6 is fair game for iMCF, depending on how slow the character is. -4 is like every time you get hit and -6 is like every they block you. And of course -2 is whenever you 2K them, making the move feel like advantage on hit against every character.
iMCFing a lot forces your opponent into using fast attacks, jump attacks or it forces them to back off.
-Even when your opponent can interrupt an iMCF, it's usually only with a select few moves. It limits their options to only fast, short ranged or unsafe moves. Depends on the character, but whatever it is, you know it's coming, and you can probably either backstep it or block and punish it. Even when your opponent is attacking from a place from where iMCF would surely whiff, attacks extend hitboxes, so iMCF will usually reach if their attack will.
-Against jump attacks, Yoshi has a few really good options. One is a:B+K, which starts the UB setup if it hits someone in the air. Another is 6K, which can easily be confirmed into the second K, which (usually?) can't be air-controlled. This does 99 damage, though 6K is unsafe. One last one is 6B, the fast mid punch, which relaunches high enough to do 6K for 50 something damage.
-If they backstep, they're at the mercy of iFC3K or 2G4KB if they're close enough. If they're not close enough, they've spaced properly. In this case, following them right away puts you in a bad spot if they decide to backstep and poke you. There's no real answer to backstep at (their) advantage except a mixup between dashing in and running a iFC3K-mid mixup. The exception is at -4 or better you can 44:bB, which is i16, and catch a CH on backsteps and convert it into a launch.
So, every time to you land one of these practically risk-free punches on CH, you can either launch into setups or bA for max damage of 69.