One of the big problems is that in space you not only have low temperatures you also have limited ability to reject heat (no real convection to speak of, only conductivity; which is bad because you can get heat seep into other parts, or radiation, or to a much lesser degree ablation). This means that it is harder to engineer parts for engines; you can have a heat range of +1000C to -200C and also subject often to uneven, and for restartable engines repeated, cooling and/or heating, which all leads to fatigue.
Secondly you have issues with fuel/oxidisers; cryogenic fuels/oxidisers need to be regularly stirred in low gravity because convection doesn't work the same as it would in the presence of gravity. This is why most RCS systems use non-cryogenic hypergolic bi-propellant fuels or mono-propellant fuels as the systems are much simpler; pressurised tanks, one or two one-way valve(s), possibly a catalyst, a combustion chamber and bell for the simplest of systems.
Secondly you have issues with fuel/oxidisers; cryogenic fuels/oxidisers need to be regularly stirred in low gravity because convection doesn't work the same as it would in the presence of gravity. This is why most RCS systems use non-cryogenic hypergolic bi-propellant fuels or mono-propellant fuels as the systems are much simpler; pressurised tanks, one or two one-way valve(s), possibly a catalyst, a combustion chamber and bell for the simplest of systems.