Heroes 5.5 WarGame Edition - Tactical Edition.

Yes, that's why griffins are considered Tier 6 underdogs. Besides them, there's not much else that could be there, but on the other hand, they are significantly weaker than their counterparts. At least, the griffins depicted in Heroes 5. Also, after Heroes 4, a connection was established that a griffin is comparable to a unicorn (in WGE, they are equal). It's not that there were many logical correlations in Heroes 4, but this particular one was liked and remembered. And a unicorn, on the other hand, is equal to a cavalry unit, according to Heroes 3. So yes, I selectively choose the reasons that I like, but there can be many "correct" options; I'm just providing the reasons for my own. Let me know if you think otherwise and that other reasons are better.

By the way, in Heroes 3, an unupgraded cavalry unit, in my opinion, is comparable to an (upgraded) black dragon in a 6:1 ratio (the stats are no worse than 4:1 + significant superiority of the dragon in mobility + significant superiority in abilities (immunity to magic)). So, there is also a basis for such an assessment of the knight to the dragon.
And also, in Heroes 3, an upgraded pikeman was comparable to a cavalry unit in a 2.5:1 ratio, i.e., the same proportions. The same ratio between crusaders and champions was also in Heroes 4.

As for the ratios of mass infantry, such as recruits and archers, in my opinion, in all parts, starting with 3, there is a serious underestimation of such warriors. At least, I haven't yet found any historical regular evidence that cavalry units would defeat second-rate infantry by the dozens (i.e., with ratios close to 1:10).

The same applies to other pairs: making different tiers, but equal in strength or growth, for creatures from the same castle, I consider illogical and harmful to the game.

And what, in your opinion, is the harm?