As a ranger I picked find familiar at first level because it seemed like the most fun option. At level three I picked the beastmaster subclass to continue the theme with pets only to realize the game won't let me have two familiars.
That was really dissapointing to me because now the feature I picked in lv 1 is basicly useless (because why would I want the small familiar when I can have the big one). I understand if limiting the number of familiars is necessary but allowing for more seems more fun, especially when you get multiple familiars from the same class. Had I known this I would never have picked them together.