I played some earlier AI War. With the balance changes of 5.0, they are nowhere near the terrors they used to be. In fact, I ended up asking for buffs either in December or January. They still do affect things a bit, but at lower difficulty ratings, you will probably be over-defending as a matter of course. They will mostly just cause you to unlock anti-dark matter turrets and mess with stealthed units and mines from time to time. The tech and economy balance makes anti-dark matter turrets accessible right off the bat.
They could be a tipping point if you're barely fighting off waves at difficulty 7.6 or 8, but I only see them being a game changer combined with some AIs and minor factions. Making any neinzul AIs even faster? Yikes. Trains while performing a shard extraction? Ugh. I don't want to imagine a golemite AI with trains supporting and boosting the accompanying fleet. Or a train master AI with the exo-galactic strike forces of the spire campaign, again.
However, in general, there are more terrifying terrain features, like Raid Engines, Interplanetary Munitions Boosters, and Warp Retaliation Posts (these less so).
Right now, the Neinzul Wardens and Enclaves are much more terrifying, because at first, they effectively ignore all your stationary defenses, terrifying your inner turtle. It takes a big change to adapt to them, but there are several methods you can develop, whether you want a hands on or hands off response, reactive or proactive, adjusting your existing defensive setup, or coming up with a whole new layout. Mostly because they ignore tractor turrets and mines, and you don't get wave warnings. They can't strike very deep, unlike the AI, but they can wreak havoc on a frontloaded defense.