'So err he couldn't have done better because there were some better sides?'
Nonense, Ferguson had some amazing resources available to him and should have done better in Europe. I personally think that in private is the only regret he will ever have at Manchester United, he didn't equal Liverpool's European record.
I think that winning two European titles with the same club is a pretty impressive achievement, personally.
Ferguson is unfortunate in that his strongest Utd side came to fruition when the Barca team of the late noughties did the same, and that is the team that many people say is the best that has ever existed. He lost two finals to them, which implies that if the best team ever to exist did not, he would have won more.
I take your point that he can and should ultimately only ever be judged on results, but you've got to look a the circumstances to a degree, at least.
Take tennis, for instance. Greg Rusedski got to the finals of the US Open in 1997 (and won SPOTY that year!). Why? Because the field was incredibly, ridiculously weak. If he was playing now he'd struggle to get beyond the first couple of rounds. It doesn't mean that he would be a worse player now, simply that what would be around him would be stronger. From that perspective, achievements need to be judged in context.
e: The thing is, I'm not necessarily advocating Ferguson as the best ever. He was lucky, at least in part, to have such an incredible crop of young, "free" players come through so early in his time at Utd, which meant he could start winning without spending, and build a bit of a war chest. But I think that his European record is better than he is given credit for, even if he let himself down at times.
