I think it's easy to criticise the Portuguese police because they didn't achieve the outcome of finding Madeleine or establishing what happened to her.
As much as people have played armchair detective in claiming they think they know what happened, so those same people will give their opinions of how the investigation should have been carried out, even though none of us are privvy to the actual ways of the police in these countries.
And to be fair, Scotland Yard haven't reached a conclusion either.
But in truth no theory should be beyond discussion, because no theory has been disproven so anything* is still possible, we are just never likely to find out what actually happened unless it ends up being a death bed confession some years down the line.
Although there is one other possibility, not of finding out what happened, but once the twins are adults, if this case stays in the spotlight until then it's entirely likely that the media will speak to them in terms of what their lives were like growing up in the shadow of Madeleine's disappearance etc. I'm not saying it will shed any light on anything, but it's an avenue the media esp the tabloids will almost certainly go down.