(There are a fair number of economists who think the Euro was badly designed from the first, Paul Krugman being the most notorious. I took my position in part from his analysis.)
You can find economists with any position on anything. Sometimes they are right. If they say that a compromise devised for the real world is flawed, they’re almost certainly right. Still not the same thing as being able to predict the future of that compromise, even if the flawed thing doesn’t work well.
It’s my understanding that the euro was instituted in order to challenge the dollar and provide a basis for a united Europe to challenge US hegemony. The reasons given by Brown for not joining were essentially that it was useful to be able to set local interest rates to counter inflation, in particular with regards to mortgages. I remember thinking that was silly when it was being explained to me in politics class, since there are great regional variations within a country (with a single currency and with no great regional variation in interest rates) in terms of inflation and capacity to pay mortgages.
I’m by no means an economist though.