Is reference better than UnityEvent ?

Actually, I have a UI which updates every time. I want to make it updating only when a player’s attribute (like HP) changes. So, I would like to know what is better: give the UI as parameter to a script or just link the UI modification script to an UnityEvent ?

It’s much better to not connect UI with update (so using a reference).

Connecting UI with events is much much better, both in terms of performance and encapsulation.

Let me show you also a trick using properties.

public class Health : MonoBehaviour
    //I'm using Actions because they're (said to) be more performant than  UnityEvents, but they are much the same
    public Action <int> OnHealthChange;

    //track and update health with a property
    private int _currentHP;
    public int CurrentHP
        get => _currentHP;
        private set
            _currentHP = value;

        //if you use a property you can be sure that the event will alwaye be called when you change the current hit points
        private void SufferDamage(int damage) => CurrentHP -= damage;
        private void Heal(int amount) => CurrentHP += amount;

public class HealthUI : MonoBehaviour
    public Health _health;

    private void ShowHealth(int currentHealth)
        //show health here, in sliders or tect
    //start and stop tracking on enable
    private void OnEnable()
        _health.OnHealthChange += ShowHealth;
    private void OnDisable() { _health.OnHealthChange -= ShowHealth; }

Further reading: Difference between UnityEvent and Actions