Comps with both Herald and Holosmith (hereby referred to as H+H comp) are too strong with no real drawbacks
What are Herald and Holosmith?
Herald and Holosmith are both multirole damage dealers. Both classes offer heavy spike damage capabilities in addition to above average amounts of CC. They both have large amounts of boon generation and utility value. They also have well above average amounts of mobility for what are obstinately brawling classes. Both classes are extremely competitive in their own right, but together they create absolute monster of a comp that isn't really counterable short of just mirroring it.
Why is H+H comp a problem at all?
Herald and Holosmith are able to synergize with each other allowing for the two together to offer a package that is immune to weaknesses that the two classes have individually.
For engaging, the Herald is able to prebuff the Holosmith with might and fury, while the Holosmith for it's part is able to provide the Herald with Stealth via Toss Elixir S. This combination allows for extremely difficult to predict alpha spikes that are also nearly impossible to survive due the absolute insane amount of damage and hard CC the duo puts out. Additionally the Holosmith is able to provide the Herald with Quickness via Toss Elixir U. The Holosmith also provides supplemental Vulnerability application that keeps the Herald's vuln-dependant Devastation traits well fed. In terms of sustain the Holosmith provides a limited amount of AoE healing that augments the Herald's largely avoidance based defenses. The Herald for it's part provides additional boons for the Holosmith. More importantly the Herald brings two AoE CCs to the table in the form of Jade Winds and Chaotic release which are important for shutting down anyone that might attempt to rescue the poor soul that is the target of the H+H duo's wrath.
What makes the H+H comp particularly unmanageable is the unprecedented amounts of hard and soft CC the comp can bring to bear. The Herald has 4 hard CC skills on 20/45/5/5 second cooldowns, and 3 soft CCs on 6/15/15 second cooldowns. The Holosmith has 3 hard CC skills on 14/15/60* second cooldowns with 2 soft CC skills on 8/9 second cooldowns. Taken together the duo can keep a target permanently controlled preventing the target any change of recovery should they have somehow managed to survive the duo's alpha. This heavy amount of CC also makes it extremely difficult to pressure the duo. Not even Stability will save you, the Herald has a AoE boonrip on Shiro and will likely be using a Annulment sigil.
Rotationally the H+H duo can dominate both teamfights and offnode fights. The duo has well above average rotation speed/general mobility which allows for easy map dominance and snowballing. This mobility also makes out rotating the pair extremely difficult. Moreover both Herald and Holosmith are fully capable of functioning independent of each other should the need arise.
In terms of matchups the pair are able to negate each other's weaknesses. A Herald might normally be extremely vulnerable to a Mesmer, however the Holosmith is able to shut down a Mesmer. The Holomith would normally be vulnerable to counterpressure from his target's teammates, but the Herald can easily shut down any counterpressure with it's multiple AoE control effects.
What makes H+H any scarier than traditional duos like Scourge + Firebrand or Mesmer + Teef?
Unlike more antiquated duos, the H+H comp has no exploitable weaknesses. Scourge + Firebrand for example rotates poorly, additionally the Scourge cannot function independent of the firebrand which limits the duo's flexibility. Mesmer + Teef on the other hand rotates superbly but is limited in it's ability to influence teamfights and struggles against team with lots of bunking power. H+H comp is able to influence teamfights and offnode fights, and has the mobility to rotate between those. The ability of the pair to function independent of each other allows for greater flexibility should that flexibility be needed.
Why block the H+H comp instead of just nerfing Holo?
Holosmith and Herald when by themselves are much more manageable. They are still quite competitive, Holosmith is particular is probably a bit too competitive even by itself, but nerfing one or both sufficiently hard enough to make the H+H comp more balanced would likely just kill the class's viability altogether. Simply put the H+H comp's power is more than the sum of Herald and Holosmith.
HOW DARE YOU IMPLY REV IS USEFUL IN ANY CIRCUMSTANCE!
While Revenant as a class suffers from nonexistent build diversity that does not change the fact that Power Herald has one of the most robust and versatile toolkits in the game and offers large amounts of carrying power to the player willing to spend the time to master the class. Granted it is a very difficult class to learn as the devs decided to design it to be arbitrarily difficult to pick up for no apparent reason, which is why you don't see many revs. That still does not change that the class is a powerhouse at higher levels of play.
*Depends on if the Holosmith is using Prime Light Beam or Elixir X for their ult.
Sanity is for the weak minded