Actually, been thinking about Gate To Hell (see, I do practice what I preach) and it may not be THAT bad
I think we focus too much on the second ability which should be thought of as a deterrent to avoid swarming it
I've never tried a Warlock Swarm build but 12 mana for a quite a resilient +1 melee upgrader may not be too bad in the right book based on 6 Firebrand Imp + 4 Dark Pact Slayer (hence not stuffed by Flame Resistance or Immunity). The Flame Hellions only exist for anti-flyer Board Control and you are playing Kill the King so despite their Flame Immunity vs. Gate's opening, too much Flame attacks is a liability against a variety of books hence the better Slayers are there instead. Flame -2 is good enough should you ever open it.
Or maybe 6 Firebrand Imp + 4 Flame Hellion + 4 Wall of Fire. Yeah that could be an interesting build, with your Warlock (Dragonscale + Elemental = Flame -4 on walls) also getting relatively safe Wallwalking. Problem is both those equipment are so prevalent in so many books and need to be Dissolved range 1 (all copies). So great in theory but the meta doesn't support it. Would stuff the Druid of course.
Demon Swarm just feels worse than Beastmaster Swarm which has Fast to counter overcommiting to an Orb + Obelisk.
And 6 spellpoints... Ugh.
I think the best we can hope for is that designers will make it playable with more cheap demons and a better spawnpoint (or sweeping or zonal damage creatures to trigger Pentagram more), maybe some sort of Demon Builder? Or mechanic that makes remote summoning (like Garrison Posts) better - but that would make Druid obscene. So they can't really go that way either...
Meantime while waiting for the support cards it needs, every time I look at Gate To Hell, it makes me think "why not play Beastmaster Swarm instead?" It's like Golems in Warlord Charge instead of Earth Wizard Teleport. There are build archetypes. Not all mages do the same build archetype equally well. I'm not a fan of Demon Swarm currently.
Yeah sorry, back to a coaster you go...