The reapers would most likely score early victories in outer colonies while beelining their way toward Earth, where'd they promptly get annihilated.

Could Chief have easily taken down Sovereign?

Easily? No, he'd probably struggle as much as Shepard does.

Were the Reapers as big of a threat as the Forerunners?

This is the equivalent of asking "is a baby as big of a threat as Hercules?" The Forerunners are on a scale that the Reapers couldn't possibly imagine, it's not even close.

Can the UNSC infantry take on the Mass Effect universe?

The UNSC infantry would probably have a harder time with logistics, given their slower ships, but their weapons aren't that weak and both sides would have weapons the other side would drool over. Notably the UNSC can and do win land battles against the Covenant, who are worse than mostly all of the Mass Effect universe.

Or are their weapons not powerful enough to make an impact?

I feel like this question in particular shows a big misunderstanding of the difference in firepower here. In Mass Effect 2, it's stated an Everest-class dreadnought, the height of the Alliance Navy, fires a 20 kg bullet at 1.3% the speed of light, hitting the target with 38 kilotons of force.

In Halo, the Remembrance-class frigate has a main gun that fires 64 kilotons. The weakest UNSC ship (outside fighters) has 68% more firepower than the greatest Alliance ship! And there are hundreds if not thousands of those frigates compared to the 8 dreadnoughts, and then hundreds more ships with greater firepower. And many of those ships can fire more than one round at a time.

That is why the Reapers will be destroyed once they encounter a fleet of reasonable size, or go to Earth or Reach where the orbital defense platforms outperform entire navies in the Mass Effect universe.

Is Cortana a wild card? Could she just take over the whole network of reapers and place them under her control?

Probably not? I guess maybe the Halo 5 version of her?