Most recently, I noticed that my iPhone is now letting me chromecast in applications where it did not exist just a couple of months ago. I feel like there's recently been a function created that allows developers to easily implement the chromecast feature. Also worth mentioning, I do not have the chromecast Ultra. That will likely play a big part in being able to stream to chromecast since the hardware specification has to be able to stream 4k.

Currently, I have Ultraflix on iOS and send it to an older Apple TV without any issues.

(Again, I also have chromecast but mostly use my Apple TV)