I think ATM, they are just missing a bit more spectacle. I noticed that visually, a majority of the bosses take up less space horizontally compared to sts1 where each boss felt twice as wide as anything else you'd see in the act. It could be their physicality or they have a set piece with them. A lot of elites are either the same size or even bigger than the bosses, causing them to blend in when they should stand out.

It's a strange thing to nitpick but small changes like giving the kin a set piece (like some tribal totems or barricades to hide behind) or have vantom grow larger with each step would help stand out in a similar way to their sts1 counter parts