The Shivans DONT suck. In fact, they were so badly pwning humanity, it wasn't even funny.
The apperance of the SD Ravana alone wiped out a GTVA Corvette, and from the briefings, it also took out another GTVA destroyer and several more ships single-handedly. The fact that they decided to risk deploying an experimental bomber against it suggested a whole lot on how desperate they were to take it out - and even then it only went out to bomber squadrons. In most capital ship engagements, the Ravana won.
When the shivans made it beyond Gamma Draconis, they managed to push allied forces all the way into Capella - at which point you can see that they were delibrately languishing around and waiting for something. The entire key here is that they pretty much could afford to ignore us due to the Sathanas's power. Hence there weren't that many capital ships needed to push into allied space. This was till the Colossus came; at which point they couldn't ignore us anymore, since we had ships with similar capabilities and firepower. And we all know the response to THAT event.
Remember - even in the Bastion mission, allied forces were engaged elsewhere, leaving only 3 Aeolus cruisers in the area. Normally, if the alliance were doing well, they'd at least have a Corvette escort around - if only to protect against larger capital vessels. In hte ending cutscene just before hte supernova we can see how well allied ships fared - a Vasudan destroyer fell to two Shivan vessels in the background of the fight, what appears to be a Lilith and a Moloch. This alone is pretty damn significant of the firepower the Shivans were throwing at the allies in terms of bombers and fighters.
The reason we see them suck so much is because we are seeing them through Alpha 1's eyes. When it comes to the Shivans, or anyone else in the universe, the player is God-like, much akin to the relation between the FEAR point man and the FEAR replicas. We simply have too many advantages that nullate the Shivan's own advantages to actually see how damn kick-ass they are. As mentioned before, try playing in insane or hard and see how easy it is to last against them.