a shower fed from a combi boiler will give better flow and more hot water than an electric shower. A typical modern combi has a power output of 30kW to 35kW. A typical electric shower has a power in the region of 10kW - so only a third as much hot water. Electric showers are always pathetically weak.
however, combi boilers are more complicated than traditional boilers, and have more in them to go wrong. When this happens you need an alternative supply of hot water. An electric shower, perhaps in a downstairs or en-suite shower room will do while the boiler gets mended.
My own preference is for a hot-water cylinder which can be heated by a gas and/or oil and/or multifuel boiler and/or solar tubes, and can also have one or two immersion heaters in it and can fill a bath or run a shower even if the boiler is broken. If you have good water flow (bucket... sink... timer..) then as well as the possibility of good flow from your combi, you have the possibility of fitting a pressurised cylinder such as a Megaflo, which gives an unsurpassed delivery of hot water. Even a combi can heat a cylinder, which is handy if you have more than one bathroom, though installers don't often do it.
An electric shower takes a lot of electricity, for example an 11kW shower will use about 50Amps. So it needs a large cable from the consumer unit, and you probably can't run two in the house at the same time. Your company fuse will usually be 80A (even if it says 100A on the fuseholder). A big electric cooker can also take 40A. Fan heaters, storage heaters, washing machines, dishwashers and tumbledriers take about 10A each during the heating cycle. So you could easily overload your electrical installation. In some districts or houses the lights may go dim when you put an electric shower on.
If you replace an old electric shower with a new one of higher power, the existing cable may not be big enough. It is not enough to change the MCB. Shower cables must never be buried in insulation as they are liable to overheat.