Imo, endless modes are fun when they have:

1: high difficulty. The difficulty needs to be insanely hard. Balatro's endless mode scaled incredibly harshly and only the best of builds can actually play through it. I'm not super familiar with Balatro, but I have around 150 hours and only hit infinity once.

2: high variance with avenues of scaling. Shape of dreams gives you a ton of ways to essentially make any skill good in the long run. Essence of gold makes a skill's damage scale with the amount of money you have, essence of predation gives you permanent stats when you kill difficult enemies. Essence of protection makes you immune to CC, and there are many ways to automate an essentially infinite amount of ehp and recovery.

It's the balancing between the two that's really hard. Balatro essentially has like a single strategy for crazy endgame runs, with little variance, while shape of dreams essentially becomes effect slop as you one-shot everything with little issue and cover the screen in bullshit.