I generally don't find this to be the case at all. Very often I find that taking a military idea group first opens up the option to take a quick boost from a powerful early idea slot much sooner than anyone else, and as long as you keep up-to-date in tech you can actually snowball harder than if you just decided to get diplo filler as your first group. In pretty much every case where I took a military idea first in all my testing, the early boost more than made up for the slightly slower start on nation specific ideas. By the time you get your second group, your tech will have stabilized to a point where you generally fall into a rhythm of tech, idea, tech, idea, so you can THEN take your diplo filler second and catch up on your nation specific ideas extremely quickly.
Well, ok, sure. We're not talking about a wholly context-independent strategy here. If your country is in a position that favors immediate military bonuses above all else, and/or if your NIs suck, and/or (as noted earlier) your ruler's heavily MIL-biased, it may very well make sense to take a Mil group first -- but those are exceptions that tend to prove the general principle, which is concerned not with your country's specific circumstances at any given time, but rather with maximizing MP purchasing power over the first few decades of the game.
In (I dare say) most cases, at least in Europe, immediate circumstances are not so dire that you
need every last ounce of extra military strength
right now. You're usually best served by letting alliances do the early heavy lifting if you start smallish, and if you start largish, you can generally avoid conflicts with even or worse odds.
To your
earlier example, Brandenburg can simply ride allies until it's big enough to take most any foe on its own. In my own recent BBurg playthrough, I scarcely fought a single battle (not counting sieges) until I'd taken basically all of the territory required for the Prussia decision (which I didn't enact, but still). By the time I turned on Poland (which wasn't gonna stay friendly anyway, given its obsession with Danzig), I could basically AFK my way through wars against eastern-tech armies. From there it's just a matter of how fast you want to grow. Vassalize some HRE electors on the side while you conquer Eastern Europe; pass some reforms; ROFLstomp through France and Spain -- and, almost before you know it,
Europe's yours.
Point being that I can't conceive of a situation in that whole campaign wherein the immediate boost from a first or second-tier Military idea would have given me a crucial leg up. Probably the best early-access military bonus is Defensive's Morale bonus, which comes second in the group -- and don't get me wrong; that bonus certainly could change the tide of a very close conflict early in the game. But chances are, you don't need to be in very close conflicts in the first place.
I've been thinking that for vassal spam SP, aristocratic might actually be a solid choice just for the diplomat. Once you're running away, you don't exactly need high army oomph.
I've taken Aristocracy a couple of times now under precisely those circumstances. It's not uncommon to end up with a glut of military points in the mid-to-late game, and an even bigger glut of vassals waiting to be annexed. Had 7 diplomats by the end of the above-linked BBurg game; 6 were occupied almost full-time.
