Here's the scenario:
The character in question has both the Tunnel Fighter fighting style from Unearthed Arcana: Light, Dark, Underdark!, which says:
You excel at defending narrow passages, doorways, and other tight spaces. As a bonus action, you can enter a defensive stance that lasts until the start of your next turn. While in your defensive stance, you can make opportunity attacks without using your reaction, and you can use your reaction to make a melee attack against a creature that moves more than 5ft while within your reach.
As well as the Polearm Master feat (PHB, p. 168), the second bullet point of which says:
While you are wielding a glaive, halberd, pike, or quarterstaff, other creatures provoke an opportunity attack from you when they enter your reach.
Assuming you hit every target that enters your reach (10 feet, unless you're using a quarterstaff), could you essentially take out a stampede of kobolds who have no ranged weapons? (Or some other instance of lots of easy-to-hit targets rushing at the player.)
Am I correct in assuming this means all approaching targets are attacked once as they enter my reach? Am I missing something that says otherwise in this scenario?
This combination seems really good to protect the rest of the group from a swarm of enemies and to abuse choke points. It's almost too good against weaker enemies.
You are correct.
Tunnel Fighter lets you make opportunity attacks for free, and Polearm Master lets you do so at a distance.
An intelligent or reasonably aware force would have a good chance to figure this out after you skewer the first few.
As for "too good," it's powerful but not unrealistic. Choke points are good strategy in the real world, too; see the Battle of Thermopylae or various castle designs that involve narrow gates or low doorways. Given that the PC has spent a precious feat and their fighting style choice on this, they deserve to have it be pretty useful.