Expand on this please. I disagree but I want to know if you thought this through.
The first - and central - point is the genre of the game. The seriousness with which we take something like the Eldrich-level threat of the enemy turns on the genre. The contrast is between WWII as seen through Schindler's List and Saving Private Ryan (the D-Day scene especially) and Captain America: the First Avenger.
Then we turn to how the "reaping" was portrayed. The sneak attack on the Citadel was central. The reapers entire plan revolves around not fighting massed armies but rather genociding isolated star systems. The mere existence of this plan - combined with the fact that the reaping is tied to a level of technological development and that development is, in theory, entirely controlled by the type of technology left behind by the reapers - suggests that the massed armies of the galaxy are a real (if not substantial) threat to the reapers. Then we have the added fact that the current cycle is off schedule. The evidence in ME1 is that the reaping should have occured at or before the time of the rachni wars. Meaning before the Turians were properly part of the galaxy. That means that this cycle has more races - more cultures - to throw at the reapers than perhaps ever before. Then we have the actual slaying of a reaper and the revelation that they have "OFF" buttons (in the form of slaying their avatars). From ME1 it looks like every single part of the reaper strategy (isolation of the species and the Citadel sneak atrack) was crushed. Then we has what humanity represented: technological innovation that did not perfectly map on to the reaper plot of technological development (we build space u-boats and space aircraft carriers).
On ME1 alone we had every reason to think that the reapers could be beaten. Especially since their numbers were totally unkown besides Sovereign's comically nonsensical line about their numbers blotting out the sky. For all we knew there were a grand total of 5 ships left.
ME2 doesn't advance the reaper plot beyond suggesting that allies from ME1 are paying off (see the rachni), that the Citadel races apparently repurposed reaper guns, and that a Big MFing Gun can absolutely kill a reaper. On the negative side is the big fleep of reapers that we see.
There's plenty of room for further handwaves to make this cycle special - like the reapers travelling a ways from dark space weakened them.
All of this is to say that by ME3 the invincible reapers were anything but that as they were portrayed. And of course ME3 starts off by having the reaper sneak attack succeed *anyway* and portrays the current cycle as being a joke compared to the last one (planets and civilisations fall in weeks). It's basically a galaxy wide nerf.
This is the Coles notes version.