Honestly, Phyrexia invading would work up until it stops being hidden political intrigue and starts turning into actual warfare. Whatever advantages Phyrexia appears to have, Ravnica basically has twice as many.
>Powerful Praetors leading the action?
There's ten guildmasters, most of them are at least on par with a Praetor for how flippin' powerful or crazy their are.
>Armies of machines
Armies of soldiers, giants, beasts, spirits, elementals, thrulls, plant zombies, mutant beasts and more. It's true that Phyrexia is known to take prisoners for conversion, but with the rate at which the Ravnicans are likely going to recycle their dead in response, it sort of levels out the advantage.
>Organized, blanketing invasion
These aren't a bunch of Mirran tribes spread out to their own lands, then forced to defend for themselves. Ravnica is a city where the presense of all ten guilds meshes throughout. If you want to take over a single district, you're going to be fighting all ten guilds for that turf at once. You can't even keep your attack plans secret anyway, because House Dimir is GOING to find out anything you're hiding.
Also, this may or may not mean anything:
>The Guildpact is in effect
This noise effects the whole plane. Whether that means the Phyrexians can be shut out is qusetionable at best, but unlikely. However, Jace can make calls on what Ravnicans are permitted to do that they normally aren't to increase the potential havoc the guilds could unleash on the Phyrexians.