Multi-layer HP bars and Buster teams

Because Merlin is mostly based on the idea of a 1 turn burst destruction and no several of them.
You see, Double Merlin at best charges your attacker's NP gauge by 40% in one turn and its Crit Buff also only lasts 1 turn.
Since Break Bars prevent a hard hitting Servant to deal 2 million damage and just kill an enemy, you to wait 6 more turns to try it again and in the meantime the enemy can kill the best Buster abusers.
Also, they now target Merlin or any support casters first so you have a time limit.
And since you can't NP loop with Buster (unless you're Sanzang) the enemy has a lot more time to destroy you.

If you have a massive one-dump chump like Berserker Kintoki or Jeanne Alter, then basically you've got a servant that will overkill the first break bar by a large margin, and waste time building up NP again in the next few break bars.

If you use an arts dps like Lancelot (Saber), then you can reliably NP almost every turn if you have the right setup. So in the end, even though his single NP does not do as much damage, it doesn't matter because the break bars mean that you need to fight for multiple turns no matter what.

It just shifts the CQ towards more about survivability and sustaining damage. Also, more future CQs will feature multiple enemies. These two together make ST one-turn-high-damage servants like Jalter and Musashi less effective. But they are still strong. Buster team also has very good AOE options like Gil and Raikou. In addition, Cu Alter has been one of the best ST servants for CQs in JP no matter how the meta shifts. I don't think Buster team has ever got outdated.

That said, quick team will become the strongest once Skadi arrives next year. Skadi doesn't offer much survivability, but makes Quick servants NP loop like crazy. You don't need any survivability if you can NP for 3 turns. We happen to have two of the strongest Quick DPS in the past two banners: Dantes and MHX Alter. If you plan to pull Skadi, it was not bad to pull them.

Arts team is decent, but has never been the meta.

It really depend on situations, not every Art teams is only useful for Spam and Stall and not every Buster team is only useful for 1 turn kill.
You can even use Zerkerlot on Buster or Quick team or even put him on a hybrid team.

I think the problem here is, peoples say Buster as '' outdated '' because they strictly follow meta and also, these type of peoples can't really improvise or adapt to the situations, this is the main cause of this ''outdated'' case.

You can divide the buffs and don't really depend on ''fast kill methods''. For example, you can use Buster Up and Atk Up for the 1st bar and def down + NP Up for the 2nd bar and for the last bar, you can going all out.

Remember, this game can be cleared by using 1-3 stars.

Basically what everyone said above—one-turn kills are not longer a thing because Break Bars mechanically prevent them.

Buster teams are still viable...but you can’t just charge your NP, stack a bunch of buster and crit buffs, and lol your way to a victory like you could in Part 1. If you farmed the Solomon raid, you likely know what I’m talking about.

Merlin, Musashi, and Jalter are still great servants. But you’ll need to think more strategically about how you use and support them because the challenge quests are only going to get longer and harder. Musashi can get pretty close to NP looping if you pull both her arts cards and Merlin + Tamamo can ensure any servant has great NP (and skill) uptime as long as you Arts chain when it counts. Jalter can NP the first turn and then you can set up a second turn of crits (Shakespeare’s 3rd skill, for example) and only have the final break bar to worry about.