Well if we are talking about the SSM let's remember that it only went live in late 2014 - there is no sense in which it was "sort of" supervising anything before then.
But I'm quite surprised that the SSM is relying "more and more" on the credit rating agencies for information. What is your source? The SSM is supposed to be able to go into the banks, go through the books and have full access to whatever information it needs as a supervisor - this is much more access than any of the credit ratings have so ought to yield a much better picture. I'm sure the SSM will look at the ratings agencies closely - again it's another perspective and also a useful insight into market reactions. But rely on the agencies rather than their own analysis? That would worry the living daylights out of me - and would be ironic given that the rating agencies received so much flack for their role in the global financial crisis.
On mortgages, debt and banking through - Greece and Ireland are often differentiated in the sense that in Ireland the banks almost blew up the government through reckless, unsupportable property lending because the hole in the balance sheets when the loans went bad was much more than the government could afford to bail out and keep the banks going. In Greece the dynamic was different as banks loaded up on government debt -normally seen as a super safe asset - but the Government had gone wild on its spending having been able to issue debt at rates it could only have dreamed of in the past before it joined the Euro. Not totally sure this adds to the discussion other than to say that the Irish and Greek cases - in terms of fragility of the banking system - had a different cause and it's not obvious the same solution therefore works.
I think you could go further and argue that, especially in 2010, there was a lot of concern that if Greek government bond holders were expected to take a cut in the value of the debt they held that contagion would spread like wildfire and all kinds of markets would be affected, many banks brought down and - unfairly because they had no choice in the matter - the person in the street all over the EU would end up potentially losing - whether their direct savings held as deposits, or the value of the assets that were supposed to contribute to pensions. And this is one of the reasons why the single supervisory mechansim and resolution mechanisms were put forward - to attempt to protect against contagion. From this perspective, I think you can perfectly well argue that the banks were de facto protected from losses as a result of investment in Greek bonds - but if so I think the motivation was much more about what would happen to the end user if everything went horribly wrong.
In theory we are much better placed to handle disruption and as many have said on this thread, Greek default has been priced into the market - though I've heard that as much as 30% of Greek government debt is still in the private sector and not the public institutions (IMF, ECB). I saw one chart (BBC I think) that suggested UK banks are still on the hook for 10bn.
But in terms of how the banks got there, I do think there was one naievty in particular that caused problems in the Eurozone - and that was that it was a real currency (and real currencies don't let sub regions leave or dip in and out) and that the risk of lending (by buying Government debt) to all members of the Eurozone was the same. The regulatory rules not only permitted but frankly encouraged this pleasant daydream. But prior to the Euro noone would have lent as freely to Greece as they did to Germany - but somehow the magical glow of the newly minted Euro and the single interest rate changed all that. And I've had arguments with people who insisted to me that "it's all in the EU, it's the same currency, it has to be the same risk." Really? How well did that work out?
The single currency is one of the biggest projects the EU has ever taken on and in 2010 the tenor of the discussion was absolutely "no-one leaves". And it was such a lauded success story of the EU, I understand them hanging onto the concept like grim death and in the face of all logic. So with this background I find it extraordinary that Germany has been prepared to even consider much less talk out loud about either full Grexit or temporary exit (which is an admission that the Euro isn't a real currency in quite the way that the Dollar or Sterling is - I don't see Wyoming ever being allowed to exit the dollar and I don't think Birmingham would be given a holiday from the pound). So whatever the EU and Eurozone were before the Greek referendum I think it is different now, but I'm very unsure of what that actually is any longer.