When will people realise that tearing up one's manifesto is part of the deal with coalitions? Now shut up and vote for this electoral reform that will ensure every government is a coalition.
It's logical that if you start with two manifestos and only have one program of government then there are going to be large sections that you can't deliver. The extent depends on how good you are at negotiating, how large your are etc.
If you don’t like this process then you are right to point out that systems like PR would lead to more of it.