On bullet spread, recoil, etc. conventional weapons being on a mech doesn't mean that it fits the same size weapons as your standard foot-soldier. Many years ago the standard assault rifle moved away from the heavier, more powerful 7.62mm round and down to the 5.56mm version, which is a pretty standard NATO size these days. 5.56 does provide a bit of a kick and forget about accuracy if you are on automatic fire.
So, you fit a standard 5.56mm based rifle onto a mech with massively increased strength and strong recoil dampeners, of course you probably won't get much recoil and can track a target with fully automatic fire.
However, a 5.56mm round would probably not do much damage to our alien antagonists, especially the kaiju versions, so a 7.62mm heavy assault rifle/machine gun would more likely be the order of the day and on the mech this could easily be handheld and fired with some accuracy from the hip, with minimum but some recoil and bullet spread on auto. This is something your average infantryman would struggle to do with any sort of decent effect.
You could even amp that up to an even larger round, say a .50 cal that can only be fired effectively from a mounted position (e.g. a mech) but would still have to be aimed to be effective as well as produce recoil and spread. You could also reduce RoF for safety and mechanical reasons as too many rounds through a barrel in a short space of time heats up the barrel, introducing barrel warp and possibly even damage to the working parts. Not to mention any gas operated re-chambering mechanisms while simple in design are notorious for clogging up with gunpowder residue and need to be cleaned constantly or the weapon will stop chambering the next round and stop firing.
Anyway, to cut a long story short, a large calibre round fired from a hefty machine gun on the mech will still produce some recoil and I do hope in Ember this will be the case.
On the omniframe, yes, I do not like the design much but my opinion is only one in many. However, what I can say is that we've only seen a couple of pictures of it and until we actually get to play the thing and see how it handles, I'll reserve judgment on it. I'm sure if, once the alpha/beta is up and running, that Grummz and his team will assess the design and if it doesn't really fit or they're just unhappy with it, they'll come up with a plan. Until then, let's get all the pieces we have together and into a playable testing environment.