Water ingestion is a cooling effect in the compressor. Without water, as each stage of compression the air heats up perhaps 20-40 C and can exit the compressor as hot as 600-700 C.
If water enters with the air and vaporizes enroute through the compressor, it absorbs some of that heat. Also, since it was in liquid form in the inlet, the mass flow is increased, so more potential power is available. All good so far.
The problem arises with the stage-to-stage matching in the compressor. The specific heat of the gas flow is changing during the vaporization, and instead of each stage sharing the load equally, some stages may be near stalling. Even this may not be a problem, because a temperature sensor between the LP and HP stages can help to compensate.
The worst case is when flying in and out of pockets of very heavy rain. The rate at which the liquid content changes can be very quick, so the interstage temperature changes very quick. If the temp sensor and the engine control system are too slow to follow the temperature changes, you can encounter a compressor stall (surge).