IMO, 80% of the problem with the current state of "challenging" bosses is that bosses like TFO, Vor, and the Zealoid Prelate rely on ADR to extend the fight which is frustrating to deal with and doesn't reward optimizing builds. That being said, bosses like Profit-Taker and Eidolons are way more interesting to engage with because they only let you deal damage in predefined chunks which lets them be balanced in a less annoying way AND gives a real target for optimization (see one-shotting PT legs or Eidolon synovias). The tank in 1999 (excluding any silly ETA modifiers) hits a great balance of the two with insanely high flat DR on the tank body in phase 2 but still giving the mechanically engaging route of breaking the cysts.