Missed forensics, less than vigorous questioning, not carrying out a reconstruction when the main protagonists were still in Portugal, letting Gerry sit with Kate with his hand on her shoulder during questioning, not questioning Kate on the 10th of May, these are just some of the PJ's actions that could have helped the McCanns evade justice. I'm not saying they did, I'm saying incompetence works both ways.
Ok, I don't have a problem with that.
What I do have a problem with is how the pro-PJ media made many of the issues you raise appear necessarily suspicious. All of which, in the absence of any PJ media strategy, got amplified by the UK tabloids, and ended up as "confirmed" because the same blurb had appeared in the UK media.
IMO, most people with with fairly firm preconceived ideas (on either side of any emotive subject) will tend to believe anything that supports a confirmation bias. Once that happens, it's hard to stand back and reconsider.