The Legionaire is still a great tank.
You're grading on a curve. The Legionnaire has always been a rather poor tank, albeit he is really the only one available. Taunts still do not last long enough and threat generation too poor for him to reliably draw enemy attention, which has been a major point of complaint I mentioned to Luke back when taunt ability cool downs were reduced.
Is it because you don't use LoS and cover in Nightmare?
Listen, I'm not saying that you cannot play him successfully. I am more than certain I can as well, but let's not pretend he is a good tank.
If your Legionnaire tank has to break LoS, that's not well designed, since one of his taunts relies on LoS. While breaking LoS and using cover is definitely excellent strategies, that really would be a tactic of an evasion tank, yeah? The Legionnaire is most certainly not an evasion tank. The DA team managed to make support/leader role characters amazing in this game, with the Virtuoso and Saarabas being recent highlights, but bloody hell can they not make a tank to save their lives. Their ongoing changed to the Legionnaire have further proven it.
How has the Legionnaire been changed since launch?
Walking Fortress duration was nerfed, decreasing overall defense and ability to keep taunts up. Tank -1
His taunting abilities had their cool downs reduced, lessening the impact of the WF nerf for taunts. Tank 0
Unbowed had it's cool down reduced, making non-tank builds with good defense a bit more viable. Tank -1
Amour was buffed, increasing overall defense without leaning on WF as a crutch for defense. Tank 0
Unbowed was replaced by a much better non-tank ability, Livid, albeit in the process we lost Unbowed as a CC breaker. Tank -1
Walking Fortress it's cool down reduction benefit removed, the primary utility for tank builds to keep the atrociously short taunts up. Tank -2
The major problem with those is that none have quite ruined the tank spec more than this latest WF nerf, but what made it worse was that the reason the duration of the ability was nerfed to begin with was primarily due to the cool down reduction of the upgrade. Now that it's gone, why it the duration still 6 seconds?
Now, with that said, this nerf would be fine if the duration of Taunts from War Cry and Counterstrike lasted a reasonable amount of time.
I mean, seriously, BW Edmonton should make a phone call to BW Austin to discuss how to properly make a tank. My SWTOR Darkness Assassin works marvelously. Tons of threat generation, two simple taunts (ST and AoE), ongoing stackable defensive buffs with tons of synergy, and an aggro dump. The two simple taunts in SWTOR have further utility by adding additional threat on top of the actual taunt effect.
My Legionnaire has one passive that supposedly increases threat (if it does, it's certainly not enough to rely on), three taunts that wear off in the blink of an eye and add no additional threat that I can see (enemies will immediately break off and attack teammates once the taunt expires), and defensive skills that have poor synergy. Good news? No need for an aggro dump skill... That happens as soon as the pitifully short taunts expire automatically.
But the worst offense? This:
Edgewise (level 24 Greatsword)
291 AoE damage
49 Armor Rating
14 Constitution
Chance on hit to taunt all enemies within 8 meters for 4 seconds.
Rune
This effectively makes the Reaver a better tank than the Legionnaire. Why are there no taunt one-handed swords or shields? It's not bloody rocket science here...
/rant
TL;DR: A tank's primary duty is forcing enemies attention on him/herself (via threat/taunts/CC) and then after that he/she should in some way miitgating damage from enemies (via pure defense or evasion). The Legionnaire fails his primary objective as a tank, but excels at the secondary. Summary: Not a good tank.