ELI5: How are the current dungeons hard?

[Jinglypockets]

But on a more grand scheme of thing, healing model in FFXIV is busted, because it's basically tuned for only using OGCDs XD

Agree.

Most of my healing experience comes from WoW, which uses a *wildly* different model in four key ways:
1. They don't have oGCDs (at least not in the way we think about it in FFXIV)
2. Encounter design involves a lot more unavoidable individual (as opposed to raidwide) damage to party members other than the tank.
3. There are very few healing abilities that can heal an entire raid simultaneously.
4. A lot more variety in playstyle between healing specs

Number 1 is a lot less impactful than people think. I think 2-4 contribute a lot more to the problems with healers in FFXIV.
Number 2 and 3 mean you have to do a lot more reactive healing, which means you can't just plan out everything ahead of time and mash Broil. Even if someone has a cooldown assigned to a big raidwide, the other healers are going to have to jump in and fill in the gaps.

Number 4 feels weird to say because the four healing jobs have pretty different feels when I play them, so I should explain what I mean. In WoW, Preservation Evokers are based heavily around using several 30s-1m cooldowns to do the majority of their effective healing. When they don't have those, the tools they have to fall back on feel awkward and inefficient to use.

This should sound familiar to anyone who's healed in FFXIV, because I just described all four healing jobs.

In WoW, Preservation Evoker and Holy Paladin are pretty much the only specs that feels this way. They cycle through their cooldowns, and once they're out of cooldowns they start to feel pretty bad to play.
Contrast with Restoration Shaman and Holy Priest which rely heavily on no-cooldown spammable heals. They have cooldowns too, but they're sending out their healing on a much more continuous, moment-to-moment basis.
Restoration Druid and Discipline Priest heavily emphasize ramping up ahead of time before big damage goes out -- similar to Astro Earthly Star or Spreadlo, but with a lot more active effort going into preparing for those big damage moments.
Mistweaver Monk, Holy Paladin, and Discipline Priest do most of their healing by dealing damage -- similar to Kardia, but at a bigger scale and, in the case of Disc Priest, with a lot more active effort to make it happen.
Mistweaver and Holy Paladin are also melee classes who can't heal effectively if they're not in melee range. Preservation Evoker has half of other caster classes' range and is heavily dependent on positioning to heal effectively.

Long story short, the actual mechanics of healing are much more involved, the damage profiles are much more varied, and there's a lot more variety in how the various healing specs play. I have fun with the DPS and tanking mechanics in FFXIV, but the healing just makes me feel sad because this is a place where WoW just flat-out does it better.