1.Yeah and most players play only main missions. What stopped Bioware from making Reaper ground froce composition more diverse? Throw some indoctrinated humans, asari, krogans with cannibals, brutes and banshees. Something like you mentioned when Saren had krogan mercenaries and indoctrinated asari alongside Geth. No, it's all Cerberus, they even overshadowed Reapers.
2.I don't see anything impossible in "winning actual battles against Reapers". Cannibals, brutes, banshees are not invincible and you could always show that without Shepard and co Alliance and other races only suffer defeat after defeat.
1. I agree that a little more diverity in the enemies would have been nice. And I originally even agreed with that having Cerberus as enemy every second mission isn't the best of writing. I'm all for having more missions with the Reapers (with improved enemy variety) as enemies. But actually have the player lose some of them (As in having to retreat or having to flee. Not as in being killed)
2. I didn't mean battle as in one skirmish with reaper ground troops. But battle as in full on Reaper assault of a particular area.
Cannibals, brutes and the like aren't invincible no. But still they have far superior numbers, brutes and banshees in particualar can slaughter entire squads before going down and for most od the fallen/captured soldiers or even civilians of your side the reapers gain another man on theirs. Furthermore even if you managed to withstand the almost infinite waves of Reaper ground forces, a capital ship could just be fed up with your antics and bomb you to hell.
And don't kid yourself. Even with Shepard the alliacne and other species suffer defeeat after defeat.
You don't actually win any major battles against Reapers in the ME3 campaing.
-On Earth the Alliance gets their asses handed to them in mere seconds and you take the only smart option and flee with the Normandy.
-On Palven your objective is to rescue the primarch, because the Turians military is bsically powerless against the Reaper assault. You manage to stall the Reaper ground forces a bit but achieve nothing to help the turians win. Palaven is later lost as Garrus tells you.
-In the Rachni mission you manage to kill/free the queen. But that wasn't exactly a battle. She was just hanging around there producing troops and no actual Reapers were involved.
-Admittedly on Tuchnaka you defeat the Reaper. But it was only one scout and again I wouldn't call it a actual battle.
-On Thessia the Asari defenses are eradicated by the Reapers. Your presence doesn't change a thing.
-In the monastary you arrive after the Reapers have already left. They already got what they came for. You are just cleaning up stragglers.
-Same with Horizon.
-The final battle on earth doesn't go to well either. Only 50% of your troops actually makes it down to earth. Then the rest of them is weakened by Husks en masse and what is left gets fried by Harbinger. And as Refuse shows, without space magic you would have lost that battle.
So no, it would be stupid to have Shepard win all battles against Reapers. Again I think that this is one of the reasons Cerberus was introduced as secondary enmy. To outsource the player victories from the Reapers to Cerberus in order to have the lore be halfway consistent (It still has some major consitency errors mind you).