The Reapers aren't harvesting or wiping out the Yahg, and they are only a few millennia away from space travel at max.
And that's one of the reasons I don't think the Reapers are an AI.
The only good thing about these Reapers in MEA threads for me is that they've made me think a little more about the actual plot in ME... and the more I've thought about it the more convinced I am that the Reapers are neither intelligent nor self-aware no matter how they're portrayed... they're basically Siri++.
First off, they appear every 50000 years. That, to me, tends to imply they're simply following a for (or while) loop - they're not reacting to how the civilisations of the organic species are actually evolving; in essence, if Year % 50000 == 0 then wake up and start harvesting. In that timeframe it's quite possible they'd wake up to find several planetary systems are nothing more than a grey goo of buckyballs already.
Secondly they're incapable of altering their core algorithm. If Shepard manages to make peace between the Quarians and the Geth, and the Geth are helping rebuild Rannoch, then he's proved that there's is a possibility for peaceful co-existence between synthetic and organic life - sure, it might not last but there is hope for peaceful co-existence. The Reapers don't recognise this ("Hope is irrelevant." to quote Harbinger), it doesn't change their core directive to harvest all advanced organic civilisations (and presumably destroy or induct the synthetic ones) - they are essentially unable to adapt to new circumstances.
Next their programming has a specific termination point. If an organic civilisation (or civilisations) manages to build the Crucible and dock it with the Citadel then a representative of that civilisation is offered the choice to destroy the Reapers, control them or somehow rewrite the DNA of every organic and synthetic species into a new hybrid form. At this point the Reapers have no sense of self-preservation; their programming has reached it's final completion point and they hand over to that organic representative. Only if that representative fails to choose one of the options presented does the Reapers harvest routine begin anew. As per the second point, the choices offered to the organic representative are immutable, anything else is beyond their programming scope - they cannot, for instance, adapt to offer a different choice if peaceful co-existence exists between organic and synthetic lifeforms.
Theoretically it's quite possible there may have been happy organic/synthetic civilisations pottering about peacefully in the dim distant past (I doubt the Geth/Quarian accord would have been utterly unique)... why didn't they survive? Were they harvested because, according to Reaper programming, those civilisations were impossible?
Finally, on a minor note, they blindly follow their programming. The Leviathan failed to include an exclusion routine to prevent themselves being harvested. Were it possible to reason with the space monsters they'd created it's possible that Leviathan could have explained the intended role of the Reapers - to prevent the subservient races from inadvertently creating their own synthetic space monsters that might endanger not only themselves but Leviathan and all other organic life as well. However the Reapers cannot be reasoned with - they only operate within their programming, attempting to reason with them is as pointless as getting angry at Windows when it crashes.
Therefore the Reapers aren't AI.
Intelligent synthetic life could have been created in the Andromeda galaxy and gone to destroy all organic life there but if that galaxy wasn't within the scope of the Reapers limited programming they'd have done nothing about it. They can't react, they can't think... they merely do a passable impression of it.