Cold water mains fed gravity hot water which is pumped to shower.

This type of system is very common especially in flats, this type of system is technically a high pressure system for the shower and low pressure to the taps, this type of system has flaws involved, the first issue is that non return valves should be fitted to wherever the cold mains water mixes with the hot water to stop the hot water filling backwards, non return valves tend to slow down the flow of the hot water, the flow is what activates the pump, so this is likely to stop the pump from turning on.

Other problems are when mixing water the cold water mains is not a constant pressure, and it will not be able to match the hot water very well causing an imbalance in pressures this can make showering temperatures unstable or even unusable.