Allowing wizards to gain advantage using an owl familiar isn't unbalanced.
No, it is not. As long as the player doesn't try to convince the DM to modify the rules so that the wizard can do this every single encounter without NPCs getting ever getting an initiative in between the owl's initiative and the wizard's initiative.
It's not advantage that's the issue, it's the gaming the system that's the issue. It's the stacking of init which in turn allows the player to prevent certain actions in the game from ever happening.
Without stacking the init, the Owl moves up to foe on it's turn and Helps, then:
a) the foe can be killed by someone else (which loses the Ow's action, although the other PC might have needed the Help).
b) the foe might have init before the wizard and kill the owl (or the wizard).
c) the foe might just move to attack someone else.
d) a different foe might do something else to hinder the wizard.
e) without being forced to ready to do this, the wizard then can still move after his attack.
f) without being forced to ready to do this, the wizard can stay behind cover or total cover until it is time to attack.
By automatically preventing these types of events between the owl's init and the wizard's init that can occur during a round, it games the system.
For one thing, it's easy for the DM to counter in game. After falling victim to this trick once, an opponent can ready an action to attack the familiar for when it approaches. Familiars have low ACs and hit points and aren't likely to survive. If the main opponent has minions with ranged attacks, it can direct them to do it. And if the familiar is killed, the wizard loses his action that round because the trigger can no longer occur.
I find this ironic. Read my signature below:
The first sign of a broken rule is when someone suggests that the way to stop it is by readying an action.
For another thing, low level wizards aren't particularly powerful, so this tactic merely redresses some of the existing imbalance.
Welcome to this thread. I have been saying that a lot in this thread.
For yet another thing, 2nd level and higher rogues can attempt to Hide each round as a bonus action. If they succeed they get advantage and sneak attack damage. So advantage most rounds isn't just for "one given class".
They can, but theoretically (because the hide rules are incomplete), they need total cover or heavy concealment to do so (shy of some special racial ability or a DM who allows hiding while being viewed while in light obscurement). If rogues can be seen, they cannot hide. Not all situations allow for total cover/heavy obscurement (or DMs who generously allowing hiding while being viewed). The reason that your "houserule" is broken is because it allows the wizard to set up his little trick every single encounter and worse, every single round within the encounter.
In my party, it's the monk that does the most damage on average, not the wizard with the help of his owl familiar suggesting that wizards with advantage aren't imbalancing.
Irrelevant. The monk does not do the most damage on a round the Wizard casts a Fireball.
This is still a munchkin tactic.