This topic comes up all the time, and there are plenty of characters out there who will rant and rave, for and against.
The first consideration is the dynamic flowrate from your cold main. The main is the only source of water for all taps, hot and cold, and loo cisterns. This consideration also applies to unvented cylinders.
Secondly, the question is, do you like a combi? Some do, some don't. There are many millions of them out there, and good quality modern ones, properly installed, are highly reliable, and should last for many years. The very best ones have a five year warranty and a high quality heat exchanger. They will give you an excellent shower with a good combi-compatible shower mixer. Fit a combi with weather compensation for the simplest and best results.