The Colossus was designed to take down ships of up to Superdestroyers in class. It was never meant to destroy a juggernaut. Keep that in mind when you wonder if the Colossus itself was a failure.
If you follow the Inferno storyline, you'll also know that the GTVA continued to make supercapital ships. The GTCa Independence and GVCa Rhemet are two such examples, both of which do not appear to be able to defend themselves properly. The Independence, in particular, has a weak flank on its port side that can be exploited (cue the EASD Nemesis).
As for the Colossus, the only weak flank I think there is is its rear.