The beast scales decently in damage, but has poor survivability. The beast also prevents you from using some combat features, like Two-Weapon Fighting.
Looking at the 3rd level – when you get the beast – If you pick a Wolf, you will have a companion with +6 and advantage to hit, dealing 2d4 + 5 damage (10 on average), the only resource spent is an action by the Ranger; considering a DEX 17 Ranger. Most classes have to give up multiple resources to achieve this consistency of damage output at lower levels. It does OK between levels 5 and 11, maybe a bit behind other classes, but when the beast gets the second attack, they become a serious threat to many enemies.
The first problem of the beast, with the current wording of the Ranger's Companion feature, is that it prevents you from using your bonus action for Two-Weapon Fighting or the bonus attack of the Crossbow Expert feat; you are not using the Attack action when commanding the beast, so these features can't proc. You can rule otherwise, but by RAW they can't be used. Weighing up in that, there's no Sage Advice or rules clarification by the game designers showing otherwise.
The second problem is that the beast gets a mere +4 HP when leveling up. Combine that with no saving throws proficiencies and an average AC, your main source of damage will hardly survive anything – it has to stay close to the enemies to hit and it will hardly make the save from AoE effects like Fireball. If you lose your beast in combat, which is very likely to happen when fighting smart enemies, you will basically lose your entire subclass for the rest of combat. If you are deeply inside a dungeon, your chances of finding a new beast is close to zero, and even if you do find one, you will have to spend 8 hours in-game to bound to it. It's tough being a Beast Master.
You can contour this kind of situation with barding, magic items and buffs from spells, but usually that is a hard price to pay to keep your subclass working without taking many risks.
Comparison to other classes and Ranger subclasses
- The Beast Master is the only Ranger subclass that can "lose" all its subclass features. If the pet dies, the player will only have its class/race/feats features available to use. It is probably the only subclass among all classes than can be in this situation.
- The Beast Master features are antisynergetic with features/spells of its base class, like the Two-Weapon Fighting style and the Hunter's Mark spell.
- No other subclass (be Ranger or other classes) is dependent on a feature like the Beast Master of its companion. You can shutdown the majority of spellcasters with an antimagic field, but they can still walk out of it and most of them have class features that are independent of spellcasting.