Background.
The First. The Lion. And also the final primarch that will be reviewed on Warpstone Flux! That's right folks, we now have a full collection of Primarch rules out in the open, not withstanding the second and eleventh.
In the book, he is portrayed as both ruthless and pragmatic which can be seen in the campaigns that he fought in the past. In particular it is noted that his function was to deal with the foes of humanity that others would fear and turn away from, much like he did in his youth on Caliban. Cross reference the xenocides that he partook in and lead.
Strengths.
The Lion, for all his rules, is actually quite the generalist to my mind. He doesn't stand out as a force multiplier (at least in the same way as Alpharius), nor is he quite at the Horus level of close combat monster. That said, his special rules give him a very good chance against most other primarchs if it ever came to it. In combat, he gets better in the sense of more attacks when reduced in wounds count. Plus, he is largely immune to certain de-buffing effects that others have upon him thanks to his Absolute Focus rule.
The stasis grenades that he carries makes for interesting shenanigans. Combined with the Fusil Actinaeus, he stands a good chance at blinding and slowing anyone he pleases. In this regard, he is quite a threat, and one that an opponent must deal with either head on, or by moving around the Lion.
Interestingly, he also gets a choice of close combat weapon: either the Lion Sword, or the Wolf Blade. The former is fractionally better for duels, the latter better for vehicles and breaking units. If you're not dueling, I'd suggest the Wolf Blade in almost every circumstance to be honest.
His buffs to his army includes a better morale check buff in a bubble, charge range (you're not going to stop him getting in to combat) and the potential for personal Death Wing body guards.
Weaknesses.
His army buffs are weak in comparison to other armies out there. Plus he does not dish out any force organization chart switching abilities at all.
Other than that, he is a primarch and that means very few realistic weaknesses.
Overall.
His points cost is reasonable for what he is able to pull off, although he doesn't particularly excel in any one area like his brothers. Sure, he's a beast, and he demands the opponent to do something about him, and he can de-buff and resist others de-buffing him. But no mass bonuses to his army. And outclassed by a number of other primarchs in the close combat arena -- but can handle a good fraction of them. He therefore sits in the middle of the power table to my mind. Dangerous as anyone else, to be clear, but more of a generalist than a specialist. Maybe that is the beauty of the Lion though.