I'd like to look at it from an other point of view, I dont think Whitelion is overpowered. I just think it does too much, so that reduces the appeal for other options
In a 24man warband you have X amount of slots, if you organize and try to get as much out of your warband you get the following from slotting a WL:
- No ramping Class mechannic. That means you can frontload channel with Fury pressed + Loner classmechanic
- Medium armor class. So somewhat durable
- MPD archtype. Acces to Pounce + Snarebreaker + Charge
- Aoe interrupt. In a Meta where coordinated channels are the combined burst for your warbands collective punch. And very good to apply procs with 1.5sec GCD
- M2 aoe dmg drop
- Nice overlapping abilities to finish/pressure singletargets to create a snowball effect or take out the enemy warbandleaders.
- Acces to 25% incoming healdebuff or armor debuff proc weapons for some utility for the warband's pressure.
Order has a proud tradition of stacking the same dps classes for orvr.
Wether it was BW for the Morale2 back before morale pumping tactics were nerfed and morales too.
Or if it was the 3 times stacking Broadhead arrow in forts.
Remember the Rampage + Retribution channeling days too?
What these periodes of time have in common is that the tempo is the same when you stack the same classes, The slayers had to go yellow then drop rage with Rampage and then climb back up into yellow while ID was ticking on top of Retribution channel. Having BWs/SW/WL on top were just not in sync. It could still work and was being done for the utility.
Whitelion didnt really get any direct buffs, they are just the "last one standing"
Slayers lost their ID + Rampage + Retribution punch despite being one of the few classes actually not camping at full classmechanic volume and going up/down for engaging gameplay. BW got -15% critdmg nerf. SW Broadhead got murdered. And what is left standing? The Lion which funnily enough does it all, and does it well in the current explosive frontloaded meta.
Try checking out how Whirling Axe scales vs Big Bouncing with 1050 str. One is a T3 monster and barely scales, while the other is slowly ramping up with gear. On top of that you get free armor pen and Ap reduction all in one super tactic for the whole build to benefit on both channel + filler.
I personally dont like WL, but I accept they are good and very very well rounded. But if we are to compare the list of why to slot a WL to a SW/Slayer it is much more clear that the WL does most of what the other options can provide to a limited warband slot, and just might be able to fit in one SW, one slayer for their unique utility such as morale drain, Numb Strike, Shattered Limbs etc. But in reality you maybe kinda wanna get as many lions(+procs) as possible.
Fun little thought experiment:
"How much more damage should a class canon dps deal over a powerhouse of a utility damagedealer before its "fair"
Does it make sense that the WL/MSH/Mara out performs the glasscanon BW/Choppa? 
 
TLDR:
Bring the other DPS classes up so their damage makes up for not having the all-around package WL has in reflection of how much utility, dmg, sustain and mobility each career has!