The role of the Speaker is to give Parliament a voice and ensure that its views are heard, irrespective of the wishes of the Government. In that regard, he was doing the job quite while. It's also entirely relevant, although perhaps forgotten by revisionists, that much of the earlier drive within Parliament for debates on exiting the EU, on holding referendums etc., came about because Bercow set time aside for the Eurosceptic MPs to have such debates and make their points. It's difficult to judge how much of an impact that had on the resulting Referendum, but it still seems likely that if the issue had been allowed to die within Parliament, then all of the fuss from outside Parliament (from Farage, etc.), would have been much less forceful in turn.
History will be a better judge of Bercow's time as Speaker than we can be at the present; at the very least, the "worst Speaker in history" accusation speaks only to recency bias.