ECB is an arm of the EU (a government, but not a nation-state) accountable, as I understand it, through the European Parliament primarily and secondarily through the Council; and it is also accountable to the people of Eurozone countries through the accountability processes of their individual central banks, since the national central bank chiefs are governors of the ECB.

Brexit demonstrates an available route to expressing disapproval of the ECB's actions.

The UK wasn't in the Eurozone - and I've been reading Yanis Varoufakis's books about his time as the finance minister of Greece and I've been fairly shocked at how fundamentally undemocratic (not to say unreasonable) a lot of EU institutions and the likes of the IMF are.

And I say that as someone who voted Remain in the Brexit referendum (note that Varoufakis actually campaigned on the Remain side in the UK).

The Eurozone nations have EU exits available to them, too.