What makes MICC cables so bad for this?
MICC looks like this:
Copper conductors in the centre, an insulator, a copper sheath (earth) and then usually an over-moulded PVC sheath. That white insulator is compressed magnesia powder, magnesium oxide.
You cut the cable to length, strip back lots of the copper sheath and the magnesia to leave you with tails. Then you terminate it like this:
The good thing about MICC, as the tradename Pyrotenax suggests, is that it has superb fire performance. It will keep working up to some crazy temperature because the copper melts before the insulation packs up. It gets specified for fire alarms on that basis alone. Also it's intrinsically safe - with an earthed solid copper sheath it is very, very hard to get it to misbehave, get hot, catch fire or spark. So it gets used in mines and other places where electricity could become a fire hazard.
That's the good. The bad is, it must be terminated very carefully. You're supposed to form a hermetic or near hermetic seal. You have to get: the pot properly screwed onto the sheath, the insulating putty properly packed into the pot, and the seal crimped properly onto the pot. If you don't make the seal properly atmospheric moisture gets in and the magnesia insulation can become slightly conductive. We're talking small leakage currents here and it's safe because of the earthed sheath. Trouble is, get enough leakage to earth building up, parallel a few runs up and suddenly earth leakage based circuit breakers can get quite tetchy about the stuff.
Also because it's not unlike rigid coax it can have significant capacitance, if that interacts with a load it can in theory be a problem, as too can, in theory, the conductor to earth capacitance, but both are rarely experienced as issues in the real world.
Edited to add: Almost forgot. When you've cut a length off the reel to use, you're supposed to seal the bare end of the reel. Most people just wrap some PVC electricians tape over it, which doesn't cut the mustard. If it hasn't been properly sealed for storage, the next length off the reel has been pre-soggyed, and no matter how well you terminate it it's going to have leakage currents from day one. Worse still, if you've done a good job of sealing it on installation, that moisture has no way to get out and the leakage is permanent.