Sustain damage vs Burst damage

Sustain tends to be better in general since they are the go-to for hard content. As far as buster-crit goes, I don't think it goes away completely, in fact Merlin made buster-crit meta throughout Shinjuku as far as I know.

As for which servants go in which team, arts teams are made up of servants with at least 3 arts cards (including NP). Best arts support is Tamamo as you said, and the best arts DPS servants that I can think of are:
- Vlad (berserker class advantages with ST damage arts NP)
- Saber Lancelot (independent crit star generator)
- Yagyu Munenori (similar to Lancelot - weaker burst damage but much greater sustain)
- Archuria archer (highly spammable NP with solid skills)
- Chloe (worse Archuria, but free at NP5)
- Nero bride (5* arts ST saber with good support skills)
- Orion for male enemies.

For buster-crit, Merlin and 2030 CE is practically required as support for the passive star generation as most of the best buster DPS servants have a hard time generating stars. For DPS, the most reliable classes are archers and riders. Any buster-based servant from either class (or with crit star gather and crit strength buff skills) would work very well in this type of team. Standouts include Ozymandias, Quetzylcoatl, Gilgamesh, Raikou and Jalter.

I haven't much played JP, but I do play with my friend's account for a bit, but anyway just an opinion. With the existence of Merlin, Buster crit is still viable since almost every turn you could create crit stars and servant skills with a 5 turn cool downs at max level that increases gather rate and such would make servants more stable when fighting. Well just based on what I hear it's still quite strong since it relies on buster cards and crit damage which is pretty easy to get off than NP specially with Merlin (lol).
The difficulties of break bars is that if a servant has good burst damage on a single turn due to his/her NP, but doesn't have good NP gain or more stable sources of damage like buster cards or crits they wont be so good anymore meaning the fall of servants like Scathach since you could burn through the first bar, but not the second one even if you overkill.
Buster crits with servants like Jalter, and such is much more...how could I say...a stable source of damage hence Merlin is so important, and why Jalter is desired by many people especially with her high attack which counts as burst damage IMO.
You could also run Arts teams if you want, but haven't experienced them so much since me and my friends are more of Buster and quick people lol.
Well for buster crits there are too many and I couldn't really remember all of them, but try to look for buster noble phantasm servants with a crit gather rate/crit strength/crit star gen skills, but more importantly Crit gather rate servants such as Gil, Archer Inferno, Sigurd, Altera, and such then just borrow a Merlin and you're set.
It would also be better with servants who has high NP gain even with Buster NPs like Altoria Lancer, and Tamamo Lancers (not familiar of others tho) and equip them with star gather rate CEs for that.
For arts teams not really that much familiar since I dont run them much, but people who has high NP gain would be ideal like Kuro, Archuria or people who could Arts crit like Empireo (Yagyuu), Saber Lancelot, but other than that Chiron, Ryouma and others don't really know, but if it's arts those things are the ones my friends usually run, not sure though since I'm not really familiar of arts teams

The multi-bar system is a challenge for Buster-Crtical one round killing. For this reason Jalter, Musashi, etc wont be so OP as before.
However, won't so OP is not means that they will not be frequently used. It is till okay for these servant since (1) they can still do another decent Buster-Critical damage after 1st hp-bar break of the boss. And (2) Merlin's broken is not only reflected for his 3rd skill.
That is reason why Merlin and Tamamo are ranked SS in Appemedia's challenge quest right now. Merlin's 2nd skill can cover the whole team for at least 2 hp bar of boss. And dual Merlin = at-least 8 rounds invulnerable from 3 NPs from enemy. Off course, when facing ST NP boss, maybe taunt unit or David/Tristan's skill is more helpful. Or Mashu and Jeanne are more tanky for the team. But what about Merlin's NP? What about 2030 Merlin spam NP? What about 4060 dual Merlin spam NP? The healing capacity is even higher than Medea Lily.

The multi-health bar is a kind of balance for restricting Burst Critical team to slash out boss in one round (even right now dual Waver+Jalter can do >700000 hp damage to Berserkers in her NPBB critical brave, let alone dual Merlin). However, in fact 3 kinds servants are even more harmed: (1) Quick servants without Skadi due to poor performance of Quick card and no configuration of Art team like Okita; (2) Amazing NP damage but not easy to spam NP like Shishou and Robin; (3) Buster DPS team without Critical, like Berserkers.

What my suggestion is:
(1) 4 supports+Nightingale
(2) Besides of focusing on damage, it is good for DPS servants have several debuff or buff skills/NP, like Orion and Vlad are more shinning in this case.
(3) Dont be so strict to Art=Sustain and Burst= Burst critical. Art critical is great for NP gain, typically Art critical for team not for DPS. like Vlad+4060 Merlin/Tamamo.
(4) Some SR/3/2/1 stars have decent ability in specific circumstances. A good example is in fact David and Tristan as I mentioned before. Even though they wont do any reliable damage but still good for team. Also sisters versus Male, etc.
(5) Critical star is very important. 2030, servants with star generation, etc. Even Instinct is also good at that moment.

Why not Hans or Waver, well because you ask other servant than Tamamo and Merlin. Both can be used on sustain and burst team and you can never go wrong with these guys.
Also like the other guys say 2030 or Crit ce support really recommended to help both the sustain and burst team.

Honorable mention :

Burst art A.K.A Nuclear team. For example Li Shuwen, Mozart, Helena, Robin hood, Gilgamesh Caster, Paracelsus. The team composition containing these guys can deal massive amount of ART card and NP damage in a single shot, Refund NP and shot another Nuclear. The weakness is can not survive longer than the Sustain or Stall art team and becareful this set up is pretty squishy, but good alternative if you don't have the Buster one.

Don't worry about the break bars too much and just take it easy. Even players who don't have Waver, Tamamo and Merlin can beat it up with just low star team composition and some don't even use broken servant like them for challenge purpose.