The main differences are:
IMCr teaches you basic attitude instrument flying, a couple of types approaches and basic nav. Other parts of the IR mght be touched upon.
The IR teaches you basic attitude instrument flying to a higher tolerance, partial panel (in FAALand anyway) competence, unusual attitude recovery, lots of different types of approaches, in depth Nav. including SIDs and STARs, procedures and rules (minima, alternate requirements, weather requirement etc...) not to mention weather, coms and more in depth theory.
The IMCr is fine for "flying in cloud" though whether or not someone with a new IMCr could fly one of IO540's TransEurope IFR cross countries?......I doubt it......Give them a year or two, then maybe.....