I also thought the assault rifles were kind of meh. But the Marksman's Rifle I found that did a 4-round burst was absolutely mauling enemies.
I only unlocked and played the Colossus, but found it to me kind of lackluster as well. Granted, I only had common weapons to use for it, since I didn't have time to farm any green or blue weapons for it. The grenade launcher was.....odd. The grenades didn't explode on contact, but had to be manually triggered, which made it clumsy to use well. The autocanon sounded fun, but didn't really do much damage.
The shield instead of a dodge was hit or miss. Because enemies often spawn behind or above you, being able to block only a single direction is....not useful. The only thing I really found the shield good for was standing in front of turrets or snipers, which are some of the easiest to avoid enemies anyway.
Overall, maybe the colossus just needs better equipment to be good.
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Actually, yes you can. The point you seem to be missing is that in an OPEN WORLD game, the walking is PART OF THE GAME'S APPEAL.
Being in the atmosphere and immersing yourself in that openness is part of the gameplay of an open world game. :/ Some games do it better than others, of course. Something like WoW's open world is a joke, and absolutely just a massive waste of time. Something like the Witcher 3 does it much better, with every little thing in the world being relevant to the main story in one way or another.