The Labour opposition to the Tories over the last five years has been the weakest I can remember. If you disagree, fair enough, but as far as I'm concerned Ed Miliband, despite being relatively likeable, was an absolutely terrible leader of the Labour party and failed repeatedly to capitalise on many unpopular and controversial Tory policies.
With regards to the EU referendum, if you believe that the UK should stay in the EU (as Corbyn does), should you go against your principles and pretend otherwise merely to cause some discontent within the opposing party? Corbyn believes that leaving the EU would be bad for Britain, he's hardly going to campaign for an EU exit, is he? There are many other issues over which the leader of the Labour party can attack the Tories, and it can be done without compromising your own beliefs to such an extent.