The 5E tunnel fighter relies more on agility than it does strength, so bigger isn’t always better.
I changed the text to adjust for the original question’s phrasing. I’ve also expanded my answer to list a few of the many ways in which that class would make an impactful contribution to a game master’s world depending on what kind of dungeons you’re running.
I made this reply 2 years ago and it is still my favorite by far! If you don’t have time or inclination to read all of my comments below, just read these two paragraphs:
The 5E tunnel fighter relies more on agility than raw strength, so size doesn’t necessarily matter as much as other classes do – where their biggest contribution to the party will be in the form of physical strength.
You could have an assassin tunnel fighter, who is adept at killing things quickly and silently, and a scout tunnel fighter, who is adept at assessing danger and avoiding it when discovered.
Since most of my answers focus on the implications of my creations for your worlds, I’m happy to see that they still resonate with readers!
Unearthed Arcana had the tunnel fighter to be able to navigate dungeons in total darkness.
Tunnel fighters get full darkvision, not just the normal 60ft that most creatures are limited to without a light source…even if they’re lit with Darkness in its obstruction sense, which means it grants no illumination for navigation purposes at all. The primary disadvantage is that you need light sources since any lights brought into an area of casting Darkness cancels out the Darkness spell already there, no matter what kind. The other major disadvantage is that this features only works effectively when conducting combat inside caves or tunnels so narrow as to allow you to run up alongside and against one wall/side for maneuverability’ sake.
Being in a tunnel is extremely limiting if you’re fighting with ranged weapons, and not every dungeon has high ceilings.
However, a character who has mastered the Tunnel Fighting style can reroll an attack roll made with disadvantage. The character can do so before knowing whether the first one was a success or failure.
The Tunnel Fighter style is for when you’re fighting very large, slow, and predictable enemies that have ranged attacks. If the enemy cannot target you effectively with a ranged attack or spell at all times (for example if they can’t cast over their head, such as when they’re too tall), this style will not work.
The tunnel fighter must station themselves either some significant horizontal distance away from their opponent, or some significant vertical distance away from their opponent. For instance, if you were in a dungeon fighting an illithid squid thing of appropriate size and hit dice… You would station yourself at one end of its chamber so it wouldn’t be able to use any spells to affect you because it can’t see over its body.
The 5E D&D tunnel fighter fighting style from UA does not require being in a light source because it was rewritten by the DM to include proficient with long-swords as its requirement.
The ability to “see through smoke” has been removed for this reason and replaced by the ability to be proficient with long swords.
Blocking tunnel fighters are skilled at absorbing an opponent’s best blows, waiting for a moment of weakness to strike. This fighting style focuses on defense and composure in the face of adversity, and a stalwart resolve that has seen them through many battles – when they do get close enough to use their weapons.
The 5E D&D Tunnel Fighter focuses extensively on defense to block out an attacker’s best shots before landing the finishing blow themselves. In order to maintain this method you must be dedicated and unmovable at all times, no matter how many times someone tries to shake your focus. The stalwart resolve required is developed early in life simply by surviving day-to-day situations with others trying their utmost hardest to break you.
The tunnel fighter’s method is about being unmovable, so why would it work in a non-tunnel environment? It wouldn’t, but the reason it was designed this way was to allow players to focus on other aspects while trying out this fighting style. Players wishing to play a tunnel fighter don’t need to be raised in a tunnel or even near a tunnel– they simply need to be set on the path of the tunnel fighter fighting style from an early age.