I think you can win vs impossible AI reliably, but you need to play different strategies than you'd use vs humans in MP. For sure, AI will try to swarm you early on. It's not that bad on choke-pointed maps like Cradle of Zen, but on open wrap-around map, like Inland the beginning is really tough. I usually go for low-attrition strategy, basically some plan that would allow me to beat AI hordes with minimal losses quite soon. Important part is to plan carefully, so that you can use one AI (or indies) as a shield vs another.