As in the other thread it was concluded that the ships (including the reapers) simply could not withstand the weapons based on how powerful they are stated by both cannon sources. Now, if the UNSC weapons were scaled down to be the equivalent of what similar weapons were like in the ME universe then yes the UNSC would lose.....
But as it stands right now a MAC round would literally rip through a Reaper. And in the Mass Effect Universe they do not seem to be very big on planetary defenses which is what ultimate wins this for the UNSC. They would have the means to use forerunner slip drives to pop up right on a planets doorstep and launch a massive barrage of nukes and jump away. The Mass Effect ships don't have this capability and as soon as they hit a planet like Earth's orbit they would be targeted by ODG platforms that all could destroy them with one shot. Not to mention the UNSC ships can also destroy the ME ships with one shot.....
And I'm not sure if the bold part can actually be proven....you are severely underestimating the UNSC here. There is no indication that there infrastructure was reduced to nothing...And saying they only have a couple useful ships is also not true. After the war Earth's home-fleet BG Dakota has at least 54 ships, including 12 Autumn class ships that have post-war tech. If you have a problem with them being able to rebuild so quickly that's with 343 but with what we have the UNSC did not seem to be slowing down until the events of Halo 6.
I don't know if this has ever occurred to you but firepower isn't the sole factor in a battle and is only relevant in a tactical sense, not a strategical. Furthermore, temporarily ignoring the post war period (I treat the entities of Pre/During War UNSC and Post War UNSC as separate entities for discussion reasons), UNSC vessels did not possess any form of energy shielding and only possessed titanium armour which was rather easily boiled off. A couple of shots from a ME vessel, especially a Reaper, would rip apart a UNSC Frigate or greater with ease.
Citation on how a MAC would "literally rip through a Reaper". Because according to physics, an object moving greater than ~1.7 kilometres per second would suffer under the effects of a hypersonic object where a projectile, while still retaining mass and momentum, would act more as a liquid in that state rather than a solid, and would violently explode on impact. If a MAC round was literally ripping through an object, yet alone a Reaper, that would strongly imply that the round is not actually moving at hypersonic velocities, and considering that the MAC is a kinetic weapon, that's actually a bad thing. If you're going to claim that a MAC round could destroy a Reaper, actually make some sense here.
No indication? I seem to recall that Reach was reduced to glass, along with numerous colonies, from the Outer Colonies to the Inner Colonies, to even the Sol System where Mars and Luna suffered heavy damage due the the Covenant Attack, though not as much as Earth, where we saw numerous cities, including Sydney, and many other locations reduced to rubble. Furthermore, within 5 years the UNSC Home Fleet had suffered heavy casualties on three occasions:
1. The Battle of Earth in 2552 where the Home Fleet was reduced to a handful of ships. Halo 2/3/Ghosts of Onyx.
2. The Invasion of Earth 2555. The Home Fleet had some time to prepare for an incoming Forerunner invasion fleet and even then the Home Fleet was reduced to a dozen ships that were almost wiped out when the Forerunner a Retrievers were called back. Halo: Hunters in the Dark.
3. The Didact's Attack on Earth in 2557. The Mantle's Approach, a 371 kilometre talk vessel is withstanding the entirety of Earth's Orbital defence grid, and is wiping out UNSC vessels left and right with its tertiary armaments. Halo 4.
The fact that there is even a Home a Fleet left is rather staggering. I would be comfortable if it was just the Didact's Attack in 2557 and the Covenant's Attack on Earth in 2552, however with Hunters in the Dark I've completely lost my suspension of disbelief (where humanity spent three years rebuilding their home fleet from nothing (Mars, the primary shipyards for the UNSC, was heavily damaged during the Covenant's invasion) when it's been completely demolished by some random Forerunner Force and forces the UNSC to rebuild their fleet from scratch).
Honestly, I think you are heavily overestimating the capabilities of the UNSC here and how much ~10 billion people can do against several trillion with many times more ships, much more Fleets, many more worlds, more widespread advanced tech (as opposed to the fact that UNSC Marines are still equipped with 7.62 NATO rounds as opposed to Hardlight rounds thats still being tested by ONI), as well as the fact that you believe that firepower is the sole factor to consider in a battle, when you have things such as rate of fire, mobility, numbers, durability and shielding, point defence guns, and much, much more to take into account.