I disagree with that first line on principle. What's more likely? That the Lion doesn't get people? Or that Nemiel just doesn't get a Primarch? That Lion El'Jonson somehow managed to get through five decades of Great Crusade on sheer luck overcoming a massive social handicap, or that Nemiel just follows the already shown trend of consciously and/or subconsciously propping his master on such a pedestal that he is shocked to discover he isn't perfect?
That entire exchange--the Primarch, the Governor, Archoi, Nemiel--was prefaced by the Lion spelling out for Nemiel that Horus could conceivably have agents anywhere. So it's not as if his quandary was so cosmic. Furthermore, the Lion asked for Nemiel's opinion after
he had already made his decision. It's not as if he was looking to Nemiel to help him make up his mind.
If we're going to use this as an example for the Lion being socially handicapped, then Rogal Dorn must be a colossal moron and/or an uncontrollably violent individual following his encounter with Nathaniel Garro. Like two renegade Astartes would really cook up a story about Horus rebelling and then try to sell it to another Primarch. What exactly did Rogal Dorn think their grand plot entailed? That he simply WASN'T going to check the facts and would instead be forced to attack Horus w/out provocation?
Ultimately, the Dark Angels seem to have been a shock invasion force. Their demonstrated tactics place emphasis on precise strikes at key locations--whether during fleet actions or planetary landings.
For myself, I also don't like terms like "Jack of All Trades". If you look at an infantry battalion, they're not "well versed in a number of vocations". They can't, for instance, jump into a helicopter or a tank as needed, and fly/drive to their objective. They can't run up to a howitzer and start laying down artillery barrages kilometers away. Their infantrymen are just that: infantrymen.
A unit is defined by its mission statement, as determined by the sum of its equipment. With that sum of equipment might come limitations. The US 82nd Airborne Division or the British Paras, for example, who simply CAN'T wage armored warfare like a US Infantry Division--which also includes armored units. On the other hand, a US Infantry Division isn't capable of parachuting in any part of the world in less than (theoretically) 24 hours.
With that in mind, no Legion* shown thus far lacks armor, gunships, artillery, spaceships, etc. Certain Legions might favor certain tactics or modes of engagement, but none of them are limited to certain sectors of war on account of differing equipment. Thus, there is no "Jack of All Trades" Legion. The Alpha Legion are great covert operatives because that's Alpharius' philosophical bent. But unless Alpharius just scrapped all his armour, he can still wage armored warfare. Mind you, he might lean more toward Rommel's playbook than Monty's (the Desert Fox famously used feints, misdirection, and allied units as bait to distract from his true master-stroke attack), but that's another matter altogether.
In conclusion, so as not to sound like a complete prick... cheers, good words, and don't take anything I said above too seriously. :D
* Though in Legion the Legio XX keeps pretty much all its assets in reserve while the Imperial Army does most of the conventional fighting.