I would've been fine if there was something in between them and the rest of the game in terms of difficulty. I didn't die once anywhere else while playing, but it took me 2 HOURS to beat Undyne and like another hour or so to beat Sans.There is nothing else anywhere close to them in terms of difficulty or design, and its weird they made the rest of the fights so dull, particularly in True Genocide where there was a lot of space for interesting fights.
Also if you didn't feel like a monster for killing Toriel or Papyrus, there was nothing that was going to make you feel like a monster ---
Moops? "I thought you were making up diseases? That's spontaneous dental hydroplosion."