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Why is that? Is this a widely held opinion?
Not sure, their Block explorer was unreliable for a long time. There are other options now, so I cant tell if they improved performance. The reused R value incident was the most recent screw up and while their reaction was good it still leaves a bad aftertaste. I think its still the most popular online wallet though.
blockchain is by far the most popular online wallet at the moment. The reused R value incident did not leave any of their customers with any actual lost funds (and blockchain actually lost very few funds as a result of this).
On the positive side, customers/users of blockchain's wallet service do have the ease of using a website to manage their funds while having the security of controlling their private keys.
With that being said there are a number of potential security vulnerabilities to using their wallet