Them being large doesn't really make them that menacing. That just makes them more distant from the player's perspective. We only ever fight their human-sized monsters and occasionally dodge the blaster of a destroyer or two. Just look at Sovereign. It ended up being kind of a joke anyway, because of its own silly tactic to embody itself in a silly hopper and it got itself killed in the process. The rest of them could have bum rushed the Citadel, they would have crippled the galaxy fairly early on.
It's their numbers that add to their menace really, not their size.
Honestly, the mimics were more menacing than the reapers.
The Reapers were handled badly in almost all regards. If a competent gamer was in control of the Reapers, then they would have won within days.
Sovereign could have started off the harvest years before Shepard became a spectre, it already had everything it needed to win. All the nonsense with the Geth, with resurrecting the Rachni and cloning Krogan... that all wasn't necessary. The Conduit? Unnecessary.
The cuttlefish controlled the most famous spectre and a highly respected matriarch. Through them it had full clearances and the authority of the council, it could have marched its pawns straight into the council chambers and triggered the mass relay into dark space. No need for an assault, no need to find the Conduit.
Even all that wasn't necessary... after all, the AI in control of the Reapers also controls the Citadel. At any point it could have started the harvest, it didn't need Sovereign's signal.
Let's assume for a moment Sovereign fails and the Catalyst is in hibernation. Let's assume Harbinger's pet project falls into the hands of the humans. It doesn't change anything.
The Reapers had overwhelming force, the Batarians were theirs for years already and they even controlled part of the Geth. How incompetent do you have to be to lose with such an advantage?
If I had been in control, I wouldn't have bothered with ground troops. I would have established my dominance in space, gotten air superiority over the planets and then covered the populations centers and military bases with air dropped Dragon's Teeth and indoctrination devices. Anything actively resisting would be bombed from orbit. After a week, the planet is mine. What do I need ground troops for? My indoctrinated masses will take care of any stragglers hiding out in the country. Or hey, I'm an incredibly advanced AI with the accumulated knowledge of thousands of species. Can't I come up with a plague that wipes out every sentient being on a planet once I harvested the majority of the population? Make clean up easier? Didn't Harbinger had such a plague tested on Omega? Why not use it again but include humans this time?
Anderson's resistance wouldn't last a week against my Reaper forces. Especially since I wouldn't give him anything to resist against. There is no need to devastate the cities. After all spaceworthy ships have been destroyed, after the orbital defenses are dismantled, after all tactical weapon stockpiles have been annihilated... there's no need for combat anymore. Indoctrinate everyone, then kill the remains with a plague, done.
A dozen Reapers for each planet, after a week the galaxy is mine. Reaper fleet was how large? 10.000 of the big ones? Even more destroyers? Against how many colonies? Massive numbers to overwhelm their fleets, then it's a walk in the park after.
Since the Reapers were able to steal the heavily guarded Citadel, why did they wait so long to do it? It would be my first move and it would cripple the council species. Have you noticed how, despite all the warnings throughout the years, they never made contingency plans? Never moved their government somewhere else? They -knew- that the Citadel was a trap set by the Reapers and stayed.
There was so much stupidity in the games and each addition was worse than the one before. Sovereign lost its shields because a remote controlled robot was destroyed? Why? The Reapers left Object Rho behind so the galaxy would know when they'd come back? Why? Harbinger was running genetic experiments with the help of its clone troopers and attacked the human colonies long before the harvest started, which drew attention to it. Oh, and was the reason the Illusive Man resurrected Shepard. Why?
Not that the council species were any better... both sides failed so hard so often, it's no wonder Shepard only won by accident.