So, just for fun I felt like rolling up a barblock and running him through a brutal solo fight at close range against an Iron Golem just to see what it might feel like in play.
Barbarian 3/Warlock 12 (level 15) vs. CR 16 Iron Golem. Deadly threshold for 1 level 15 character is
6400 XP, this fight is
22,500 XP, so this is
3.5x Deadly.
Note that the Barb will be fighting recklessly (in all senses of the word), not using his spells to full advantage (e.g. no Darkness), and gains no real benefit from his GWM feat because an Iron Golem's AC is too high. He also gains no real benefit from Dark One's Blessing because it's a brutal solo fight. This fight is kind of a worst-case scenario from the axehappy Barbarian's perspective--it's meant to be basically a test to destruction.
Barbarian wins initiative
1a. Barblock casts Armor of Agathys V, readies his polearm, and retreats slightly
1b. Iron Golem quickly advances into melee range. Takes
13 damage from polearm opportunity attack, cannot attack this turn due to Dash.
2a. Barb rages and recklessly attacks Iron Golem (NOT power attack, no bonus attack). Inflicts
27 damage with two attacks.
2b. Iron Golem attacks with sword. Hits twice for
24 points of damage. Takes
50 points of cold damage.
3a. Barbarian attacks three times, hits thrice for
51 points of damage. (First hit is a critical so GWM replaces PM strike this turn.)
3b. Iron Golem breathes poison. Inflicts
24 points of damage (failed Con save), which polishes off the last off the Armor of Agathys.
Barb is down
23 HP, Iron Golem is down
128 HP. Barb is mad with bloodlust, will not disengage to refresh AoA or anything. Now they both just hammer at each other until someone dies.
4a. Barblock hits once for
23 damage.
4b. Iron Golem hits twice for
16 damage.
5a. Barblock hits twice for
33 damage.
5b. Iron Golem hits twice for
30 damage.
6a. Barblock hits once for
12 damage.
6b. Iron Golem hits twice (one crit) for
36 damage.
Barb is now down
105 HP (out of 131). Iron Golem is down
196 (out of 210).
7a. Barblock hits for
32 damage. Iron Golem dies. Barbarian gains
16 temp HP.
Barbarian now has
26 HP + 16 temp HP remaining, probably wants to take a short rest.
Admittedly, this is the stupidest possible way for the Barblock to engage the Iron Golem. The fight would be smarter but a lot more boring if it went: Ascendant Step (Levitate) + Hex/Eldritch Blast for 11 rounds in a row until 222 HP of damage are finally inflicted. Or even if he had avoided raging, relied on Blade Ward, and just kept recasting Armor of Agathys (using Ascendant Step as necessary to withdraw from combat to refresh). He has a lot of options which I didn't use in this fight because I felt like being a MIGHTY RECKLESS BARBARIAN for once, and he came close to dying, but the fact is that at level 15 he soloed a CR 16 monster in melee in the most direct way possible and he still survived.

I'd call the concept "viable".
[Original stats: 10 11 16 15 8 15]
Trog, Half-elven Outlander shaman, Barbarian 3/Warlock 12
Str 18 Dex 11 Con 16 Int 8 Wis 10 Cha 18 HP 131 AC 15 (Half-plate) or 17 (Half-plate and shield)
Polearm Master, Great Weapon Master, +2 Str
Thirsting Blade, Devil's Sight, Agonizing Repelling Blast, Ascendant Step, Lifedrinker
Athletics, Deception, Intimidation, Stealth, Insight, Religion
Fireball, Fire Shield, Darkness, Wall of Fire, Hypnotic Pattern, Counterspell, Hex,
Expeditious Retreat, Armor of Agathys, Hold Monster, Dimension Door
Eldritch Blast, Blade Ward, Minor Illusion, Friends
Mass Suggestion