MysticalOS, the creator of Deadly Boss Mods, posted a video highlighting the issues with Blizzard’s built-in boss timeline, covering the bugs on each boss, and how it’s failed to meet its goals.
As part of the addon pruning heading into Midnight, a design goal of Blizzard’s was to add more of the tools addons provided into the base game, granting players more accessibility. One of these tools was the Boss Ability Timeline.
The Timeline was a feature added to Raid and Mythic+ bosses, and gave players access to an in-game timer for boss encounters, allowing players to plan ahead and prepare for each boss mechanic. Previously, this option never existed in the game by default. Addons like BigWigs and DeadlyBossMods (DBM) provided that overlay for raiders.
MysticalOS, the creator of DBM, made a video about the boss timeline, highlighting the lack of quality control on Blizzard’s end and how addon creators like him are filling the gaps left by Blizzard.
Breaking down boss by boss of the tier, only two bosses, Chimaerus and Belo’ren, had no bugs. Certain bosses have smaller issues, such as abilities miscounting like 
Grasp of Emptiness on Crown of the Cosmos.
For other bosses, larger bugs arise that can severely harm progression and mislead players.
In a couple of instances, the Timeline will sometimes queue an ability that won’t trigger because of phasing. This happens on both Vaelgor & Ezzorak, and Fallen-King Salhadaar.
In a couple of others, the timings on major mechanics are off on the Timeline, or missed altogether:
- On Mythic Vaelgor & Ezzorak,
Dread Breath (the frontal cone that fears everyone in it) timers are off by three to four seconds during Intermission, either overshooting or undershooting. - On Imperator Averzian, the 2nd soak of
Umbral Collapse doesn’t show up on the Timeline, a core mechanic of the fight.
In both instances, doing those mechanics wrong will almost definitely lead to a wipe.
Failure in Features
MysticalOS continues on about the failure in features, specifically the lack of customization options for players. Users can’t toggle abilities to see on the bar, which results in information overload. The example given is that DPS will be given tank busters, which aren’t relevant information.
Players get a lot of information, but at the same time won’t be hit with warnings. Blizzard only uses the ENCOUNTER-WARNING API with only certain mechanics in a fight. The API itself actually has the ability to show 3 warning levels and options to control which ones, but Blizzard’s design has ignored this feature, opting to show users what it thinks is important, and ignoring several important mechanics.
These failures in features led to the failure of their goals. The lack of player agency and features means that addons are still mandatory. Addons are filling the gap. DBM, Northern Sky, BigWigs are still used commonly by raiders instead of the base feature. Secret APIs have made their jobs a lot harder as well, but their addons give more functionality than the base timeline.
Deserving of Praise for Private Debuffs
MysticalOS offered Blizzard praise for making debuffs private. They’ve removed the computational nature of addons, and players can no longer use macros as a workaround. Midnight Falls is excluded from this, due to its difficulty.
In Dragonflight, several notable fights had these workarounds. For example, Smolderon from Amirdrassil had this with 
Seeking Inferno on Mythic difficulty. Orbs would lock onto players, and players had to quickly hit each one, but they could not be touched at the same time. Players would create a macro, and by pressing it, a list would appear of who had orbs, solving the mechanic.
