My expectations are high, but also reasonably skeptical.
Thought this has less to do with their power level and more to do with concerns about their roles, designs, and feel overall due to Oda's track record. I'll touch on that more below.
Power Level
I expect them to equivalent to the strength of at least 1 if not 2 World Powers collectively as a unit or faction. They have to be stronger than the Revolutionary Army overall or at least comparable to them with Dragon taken into the equation. They should be more well-rounded as a squad than the core members of a Yonko crew, even if their strongest member or Grand Master isn't on the same level as an Emperor himself.
We currently don't know much about their hierarchy besides the fact that they have 9 members (not unlike the 9 Black Riders/Nazgul from Lord of the Rings) which mirrors the Straw Hats. We know that the Straw Hats have 10 members including their captain, so it's possible they won't match up against Luffy himself.
We know that Garling is their "supreme commander" or as I like to call it, the Grandmaster of their order. We also know that he was the former king of God Valley. Thus, I believe it's reasonable to speculate that he is comparable to some of the old legends now. However, we do not know for sure if he is their absolute strongest member, or if there is a younger in their prime colleague of his who actually serves as the powerhouse for the Holy Knights (the silhouette that resembles Shanks). If the latter is true, then we should have at least 2 members whom are comparable to Admiral level if not higher (including Garling).
Ideally, the Holy Knights should be > the Red Hair Pirates, with their strongest member and the other 7 members mirroring the captain and crew without even taking Garling into the equation.
I hope this would be the case, and that their 3rd strongest member overall should be no weaker than an elite YFM (Beckman)?
They are the justiciars of Mariejoa and the judicial force of the Celestial Dragons acting as their mediators of the Holy Land. Dragon seems to believe that the fact they are becoming more active and making moves is somehow a big deal, so this once again reinforces their formidability as a unit.
Overall, I expect them to be strong enough to solo a Yonko crew as a collective. Thus, they may or may not exceed the Admirals' title of "World Government's Strongest Military Powers" as a trio which would help keep the consistency of the Government's military hierarchy.
Characters
This is where I have more skepticism and questions as far as the Holy Knights are concerned. I have no doubts they will be powerhouses (maybe even individually), but will they deliver as antagonists? Will they satisfy the expectations as actual villains from this so called Holy Order where they effectively serve as Praetorians for the CDs?
Will their designs be well rounded and great enough where it matters for them to be taken seriously?
I have legitimate gripes with these points because Oda hasn't necessarily delivered so consistently in this regard in the past. Usually, this only applies to a Big Bad and his next best man (see: Kaido & King, Big Mom & Katakuri, etc.).
He failed miserably with the Wano Country arc in regards to the Samurai faction as a whole. Besides Oden and arguably Denjiro (whose turn made him more pathetic), there isn't a Samurai who checks those boxes outside of a future version of Momonosuke. Likewise, Orochi was absolutely horrible as it pertains to expectations of this big bad Shogun who acted as Kaido's ally. The Beast Pirates themselves along with the BMP miserably failed to deliver or stand out as anything more than standard arc villains.
Furthermore, Oda's penchant for sticking to largely to the proportion of more eccentric, goofy, and over the top villains gives me pause. I don't want 7 of the Holy Knights to be a bunch of Queens or Orochis in terms of personality. I'm not saying they need to be edgy, but they should radiate badassery.
There are also cautionary tales outside of the series itself when expectations of some enigmatic, all-powerful faction are at an all time high. Look at the Royal Guard from Bleach for example. While they satisfy the power level requirements of their station, only perhaps 1 of their brood stood out in the design department.
With that being said, I do like Garling's overall looks, design, personality, and introduction or role within the story with what little we've seen of him. So at least one of their prominent members check those boxes effectively.
Time will tell if the other Holy Knights deliver in this department.
