My experience of Combis over a 20 year period is as follows:
1) They don't seem to last long - first one lasted 7 years, next one lasted 10 years and we sold the house with the 3rd fitted. Reliability not brilliant - but skill of engineers not either. Cost of new boiler installed was viable when stacked against the ongoing repair costs.
2) No pressure probs with hot water as had incoming 22mm mains supply which then provided 15mm cold feed to all cold pipes and another 15mm feed to the combi for hot water.
3) Had to top up pressure on all three of them - some more than others. Gas engineers reckoned it was minute leakage in the motorised valve switching between hot water and heating. The heat within the boiler would then evaporate the weepage leaving no sign.
4) Had one spectacular failure though. You know that pause where you are ready to go out - stood there in your coat and waiting while she faffs about. I hear a loud crack and then the sound of water running and then spraying inside the boiler. A plastic bodied valve carrying mains water had split. Would have flooded the house had we left seconds before. No lasting damage and a fairly easy fix.
5) Have a conventional condensing boiler now and much happier.