HQs can generally be split up into 2 categories; those that support, and those that are supported. The latter is made up of your beatstick characters that go out and do some damage for you, whereas the former enables other units to do the damage. Of course there is some crossover between the 2 but if a HQ choice is not particularly efficient in a support role then it is generally shoehorned into a beatstick one. Maugan Ra has a similar problem.
The problem with him as a support character is that he doesn't really add much. He has Relentless, which is normally huge but with Eldar not so much. The infantry units in the Codex generally have Assault Weapons anyway, the only real exception being Dark Reapers who are already Slow and Purposeful. The only other real support rule is Night Vision, which is ok. The problem being the short ranged nature of most Eldar infantry weapons, again with the exception of Dark Reapers, who can just buy it for minimal points anyway instead of blowing almost 200 on a Pheonix Lord with it. The only support perk really being his ability to tank wounds quite well, being a 3 wound, Eternal, 2+ save model. However any of the Pheonix Lords can do this for you and many get you more for your points.
So, Maugan Ra doesn't fit a support role particularly well given his points cost and the level of competition when it comes to support characters in the Eldar Codex. Individual ability may be more his style however, after all, he did solo a Tyranid Hive Fleet and pull an Eldar Craftworld out of the Eye of Terror.
To an extent, he does function a little better as an individual. As has been previously mentioned he has an extremely strong statline, in addition to Eternal Warrior, Fleet, Fearless, etc. In addition, he also has Relentless, which isn't fantastically useful given that he is wielding an Assault Weapon anyway, however the weapon in question is pretty nice. It's 36" range, which when combined with Battle Focus and Maugan Ra's normal movement is pretty good. It's also S6, and although it is Ap5 it is Rending and Pinning. It can also churn out 4 shots at BS7 every turn, which upgrades to 5 due to his Fast Shot rule. Finally, because it has a big scythe on the end of it, it counts as a close combat weapon. It gives +2 strength in close combat and is AP3, which is ok in most scenarios but could be a problem against Terminators, who would probably destroy Maugan Ra in close combat due to his lack of an invulnerable save.
And there's more, Maugan Ra has Marksman's Eye, allowing him to Precision Shot on a 5+, especially useful given his weapon's Strength 6 against all of your Ethereals, Haemonculi, etc. And then he has Hatred (Chaos Daemons), which isn't bad, it just relies not only on you playing against Chaos Daemons (not unlikely in today's environment) but also on Maugan Ra being in combat with one of them.
Finally we come onto the Warlord Trait, which gives him Split Fire, which actually suits him. What it does is allows you to put him in a squad, and then shoot at different targets to it, as Split Fire only allows a single model to fire seperately from the squad. This is quite fitting, as it allows you to give the squad that Maugan Ra's with the few buffs that he actually has, yet also allows him to act as an individual.
However, as the buffs don't really add up to too much, you have to justify his points cost mostly on how he performs as an individual. Is a 5 shot S6 AP5 Pinning, Rending Gun at BS7 Precision Shotting on a 5+ with some counter assault potential thrown in worth almost 200pts? I'm not sure, but if I had to make a call I'd probably say that I wouldn't take him. There's too much competition and too many other cool shenanigans in the Eldar HQ slots to justify Maugan Ra in a competitive army. However, if anyone else has worked out an interesting combination for Maugan Ra that is working quite well for you I'd love to hear it, so please comment.