Lots of builds are "viable". You can play a Dex-build Halfling barbarian if you want. The issue with bladelock is that there is no build as-designed that's as good as the other classes.
A mobile blade-lock isn't especially mobile; they cannot Dash/Disengage as a Bonus Action like a ranger or rogue can. Trying to play a straight blade-lock as a mobile striker will be an OA magnet.
A tank blade-lock doesn't have the armor every other front-line class gets automatically. You can fix this with Feats, but in the mean time a real Paladin or Fighter is using those same ASI/Feat slots to pull further away from you with Heavy Armor Master or whatever. So you cannot ever be as good.
Whether you go mobile or tank, as a class that relies on spells for damage, getting into melee is asking to have your Concentration disrupted. When you only have two spells per short rest, you cannot afford to re-up a lost spell the way a Cleric or Druid can.
Honestly though any of these problems can be fixed by expanding the Invocation list just a little. Armor of the Fiend for an AC that scales with level. Tower of Iron Will for advantage on Concentration checks. Slippery Form for Dash/Disengage. Scale blade attacks at the same rate as Eldritch Blast. Etc.
It wouldn't take much to bring blade-locks to par with other classes. But as-written there's no point trying to argue they're on equal footing. The case just doesn't close.
EDIT: I should make clear that I really want blade-locks to be better because I find the standard "hang back and Eldritch Blast a lot" to be boring as sin.
Thanks for your perspective. And I could not agree more with the boring nature of hang back and blast ALL of the time. I agree that they do not have the armor. Note in my original post I stated this is a problem if you want to play front line.
However, whether we use feats or Dwarves I say that an armored blade pact warlock is within reach. As to front line, is a blade pact warlock less able to swing a weapon and stand alongside a cleric a ranger or fighter? They may not do as much damage in this part of combat as a barbarian or paladin. Again, I am not sure if this is a problem.
Wearing armor, I can at a later point in the game levitate and blast down, use illusions with impunity and perhaps even be invisible. Perhaps I have mask of many faces. Yes, I get these abilities for a price. I can also potentially get two attacks a round and bring some hurt with hex and lifedrinker/thirsting blade when level appropriate.
Again, my whole point is this: If we assume no armor upgrade with a penchant for front line fighting there is a problem. I would not enjoy this likely frustrating enterprise. But gaining armor through feats or race, a blade pact character is viable and versatile with the ability to fight without multiclassing. Not at the level of the fighter, but a fighter cannot fly, see in magic darkness or be invisible either.
If we compare the loss of a "feature" familiar or Tome to gain two attacks with a heavy weapon summoned at will even if captive which can ultimately do charisma damage, I still assert you are not giving something up for nothing. Aside from a few martial classes being superior, you gain the ability to fight better than most. That to me is something. Hell, you could drop eldritch blast invocations entirely and have an armored warrior that can create absolute chaos through illusion, disguise self and invisibility alone. You could even misty step up to the bad guy and drop the hammer (or greatsword). A lot of pure fighters would have trouble getting there that fast without being encumbered along the way. And you might have a better shot at shrugging of the BBEG's spell with your Wis save proficiency.
I am not saying that you cannot find a higher damage build. I am not saying that you cannot be more efficient by dipping and cherry picking. But I am saying that a single class blade pact warlock fully armored has a lot of options and strengths and is on par with many other classes. For versatility they are at the top of my list.
