WOTC acknowledged in the preface of Unearthed Arcana: Revised Ranger that the Ranger Archetype beast master has been unpopular to play as it is underpowered.
Many players want to play rangers, but few were happy with the class, which held its place at the bottom of class power rankings by a significant margin. The class’s individual features also filled the top-tenlist of lowest-rated individual character features. UA Revised Ranger
They first attempted to rectify the issue with specific changes to the animal companion in the Revised Ranger archetype Beast Conclave allowing the companion to attack on its own among other changes.
The companion may take its own initiative and turn controlled by the player. The companion also gains the following features in addition to its own attack: at level 5, it may attack as a reaction to your attack; at level 11, it may attack multiple enemies during its attack on its own turn.
Coordinated Attack Beginning at 5th level, you and your animal companion form a more potent fighting team. When you use the Attack action on your turn, if your companion can see you, it can use its reaction to make a melee attack.
Storm of Claws and Fangs At 11th level, your companion can use its action to make a melee attack against each creature of its choice within 5 feet of it, with a separate attack roll for each target. Keep in mind that the base Revised Ranger class does not get an extra attack at level 5.
With the later release of a new unearthed arcana, UA Class Variants revised ranger, it improves again on the original PHB ranger archetype. The appearance of these new options inform us of what may have been undesirable in the original Ranger.
In the newest Ranger, hunters mark is a class feature rather than requiring a spell slot and additional spells are provided such as class features, Speak with Animals for example not requiring the use of a spell slot.
This indicates that the original PHB Ranger was underpowered in spell casting compared to other classes. Ranger as a half caster with limited spell slots was hampered, utility spells like Speak with Animals were not being selected in favour of more powerful spells like Hunters Mark.
The new Ranger is able to heal their beast companion at the end of combat if they fall unconscious, whereas in the original the Ranger would have to spend 8 hours magically bonding to a new animal.
New beasts were added, "Beast of the Earth" and "Beast of the Air" which are significant improvements over many individual beasts of challenge 1/4 or lower. These new options allow a Ranger to use one stat block to represent any number of beast appearances, giving a more versatile option to the player.
In all the revisions some things have remained the same with the Ranger class, the progression tables have remained unchanged indicating that this aspect was not in contention, as well the extra attack and fighting style features were unchanged.
It appears dissatisfaction in the class was related to the survivability of the beast companion, the number of available spells per day and specific class features; favoured enemy and natural explorer.