The economic austerity measures: reduction in government spending,  increased taxes usually come after the economic stimuluses inflate the economies and bail out corporations. In some economies the economic stimulus may not have been enough and the economic austerity measures came too early. 

What will happen in the UK and other nations which implemented harsh economic measures too early?

economic theory generally states that a severe cut in demand (which is what cuts in Gov spending and tax rises do) in the economy could damage growth or cause a second recession for the UK.

Whether that happens, is still completely up in the air

Couldn't be any worse than now.

I don't think it will have negative effects.

In the UK, the things that are being cut right now are mostly frivolous (there was so much waste in the budget left by the last government ) and didn't contribute to useful economic activity (various quangos, defence projects, school construction, Child trust funds, the £25k excess earnings allowance on tax credits (i.e. legalised fraud)).

If it was health, police or education jobs being cut then I would agree. But we don't know of any yet.