Safari Zone locations, Roaming Legendaries, Teleporting & Roaring Pokemon.
We've always have had Pokemon who were able to escape battles (let's not forget about the Pokemon who can just kill themselves by exploding or using one of the other self-fainting moves). However, Let's Go bring in the GO mechanics which allows any Pokemon to escape an encounter when they feel like it not to mention having you able to miss throwing the Poke Ball (which was only in Gen I and only against the Legendary Pokemon with high HP).
While a discussion probably better suited for the Let's Go thread, I'll just quickly say while it works perfectly fine for GO (which is meant to be played on the go, thus makes sense why you can't wait to have a few minutes battle as you chuck as many Poke Balls as possible), for a console game I'd be sitting down to play I feel it's much more irritating. As fun as throwing the Poke Ball sounds, if there's a chance of me missing a throw I'd just stick with the plain ol' "press A to throw ball". Let the Pokemon's catch rate be the difficulty, not my throwing skills.