Of course the cheap ones are bad news. I never suggested to use the cheap ones.
IDC is definitely less desirable than crimped. IDC involves slicing through, displacing insulation, and relies on a "spring" effect to maintain a connection to the wire. Conceptually:
This sort of terminal is prone to bad things happening because of mechanical stresses on the wire causing shifting and variations within the terminal, plus the area of the terminal that is actually in contact with the wire is fairly minimal. In the telecom world, generally IDC connections are only considered for solid wire (66, 110, etc) because they're so effing prone to failure with stranded. If you have a right-angle SATA power connector with a little plastic cover on it, that's probably an IDC connector like this:
That's a shitty prebuilt modular cable. The pins used are actually a dual IDC which pierce the insulation in two different places.
Crimped is a whole different ballgame:
where you actually have a large section of the terminal crimped AROUND and into the stranded conductors, plus another section of the terminal wrapped around and grasping the insulation, forming one of the best connections you can get short of soldering.
This is what it looks like:
But the thing with these is that they're still somewhat prone to individual pin rip-out, because there is only a metal or plastic tab holding them in place. By comparison, the moulded ones are held in place by moulded plastic. The good quality moulded ones are by far the toughest customers as connectors go.