I can speak from some of my AI experience in 1v1 and 2v2 against mainly mediums, what works and doesn't work in my experience.
I echo the earlier sentiment that sometimes you just lose flanks as this faction. It happens. However, something that is very nice in Phase A is the Hs. 129, which can easily kill the Churchills or any other tanks in the phase. It is hard to counter this aircraft except with other aircraft. However, the Me 109 is a very good fighter for protecting your bombers. This is important because the Hs. 129 is your primary defense against armor, and it can even kill Jumbos frontally thanks to the way the penetration system works, so find ways to use this aircraft effectively. You can get more in Phase B and C. Force your opponent to invest in AA, and then shut down that AA with the Fw 190 bomber and your infantry. Or use bombers to shut down infantry pushes or blind fire on artillery. Use the Me 109s with veterancy to wrest control of the skies. If you can get air superiority, life becomes very, very difficult for your opponent. The Fw 190 fighter/bomber is a very powerful unit that is not to be underestimated.
As for the ground forces, well.. the 55 point H35 tanks are basically the Erzatstruppen of tanks, and can be surprisingly useful if used to ambush or push unsuspecting opponents. You can get a good amount of Fallschirms to hold cities and bocage, and with 3 vet thanks to a cheap commander that has AT--these guys will melt enemy trooos. The 91st also get the IG18, which can shut down infantry pushes in the right circumstances. Mortars are still mortars and they can be used to provide smoke cover to set up ambushes with your Fallschirms. The Pz. Abwehr Recon are also quite effective units and 2 star Spähtrupp are no slouches, either (cheap commanders again help).
Erzatstruppen melt under any sustained fire, but can be very useful for holding territory in Phase A with proper support from AT guns and a commander. I find them to be good forward suicide scouts since they are so cheap, but they are surprisingly durable in buildings and heavy cover. The phase A 75mm Pak can also be very useful in a good position, and the 75mm can shoot HE shells as well. Be careful for arty, of course. The 37mm AT gun is occasionally useful, but mostly just cheap--you'll use them not because you want to, but because you have to. Like many other things, try to keep a cheap commander on hand at all times.
Phase B is arguably the roughest for the 91st. They get access to very middling ground troops during phase B and must make a bid for air superiority during this phase. The Marder II can be effective if used correctly, as can Grenadiers and Pioniers, but your forces will wither without proper air support, especially from either Fw190's for air and ground support and Hs. 129's for AT. You can get more Pak 40's in this phase and you'll want to use those to good effect. The Panzer III is more a support unit than anything else and shouldn't be counted on.
The arty for this deck is middling. Rely more on air support and use artillery to deceive your enemy with smoke, or use it to blast AA so your planes can do the real work. In phase C this deck has a bit of a resurgence, especially if you can get air superiority. Ju 88's are virtually impossible to shoot down without fighters, and by now you can get even more Hs. 129's. The 380mm battery in this phase can be devastating, but also it's phase C, so it only matters in close games. You also get a decent StuG III in this phase, so that's neat.
This is by no means a comprehensive list, just some observations I've made based on my experiences with them thus far. Since we're so early still, I could be completely wrong, but these are some thoughts.
A final note: I think this team works better in team games since aircraft can be force multipliers outside of a single flank. If you're directly communicating with your teammates you can coordinate air strikes and even have your buddies cover your Hs. 129 with fighters of their own.
