After thinking about this for a while now, I think I'll have to say "no" on both counts, at least with respect to cash-rushed armies.
That the AIs won't attack armies does come as a nice advantage to them. But, the AIs also won't attack a veteran infantry, and an artillery proper unit inside a fortress (and you could also set up a funnel of doom/other army walls tactic this way also... the infantry and artillery inside a fortress get a "free shot" also, and you can shoot with the artillery every turn also). So, armies don't end up overpowered in this respect, since there exists a potentially cheaper way to get the same effect... at least in many places on most maps, just not as early in the game.
Cash-rushing an army costs you 1640 gold every turn, or a little less than 1560 gold everyturn in a 1 shield city. If you don't have Leo's you can upgrade 10.4 horses to cavalry for 1560 gold... or if you have a bunch of 1 shield cities with barracks, you can buy a horse every other turn for 116 gold, upgrade for 150, and put out about 5.86 ((266x5.86)=1558.76) veteran cavalry every other turn. If you have Leo's, the same process yields you about 8.16 ((191x8.16)=1558.56) cavalry.
If you face enough backwards defensive units (spears, pikes, and maybe muskets too... though that seems like the toss up call), those cavalry actually end up a better value, in my opinion so far at least, than a cash-rushed army. If you don't face backwards units, but have a lot of well-positioned artillery type units, those cavalry will also probably do more for you. If you face rifles and infantry, without well positioned artillery type units, the advantage probably goes to armies... if you have multiple wars to fight... but if you only have one that might work out differently. So, armies might end up better here, but it comes as at least close.
The benefits of cash-rushed armies may come as easier to take advantage of earlier in some situations. No doubt though cash-rushed armies do have advantages in many cases over other ideas. But, it seems that those gains in general don't seem too overpowering... so long as you don't mind disconnecting and reconnecting for upgrades and/or building some fortress.
Armies spawned by leaders though come as another story, since one might argue you pick up the equivalent of several cavalry with an MGL for cavalry. And armies when the AIs have enough bombers also comes as another story.
