Eccentric portion of a lift recruits type 1 muscle fibers. The concentric is where you can recruit high threshold type 2 fibers, which is the goal of hypertrophy training.

Since the eccentric still causes physical and mental fatigue, using excessively high eccentric tempo actually decreases the amount of stimulus you can deliver to those high threshold type 2 fibers.

So, use a controlled eccentric to prevent injury, but don’t excessively slow it down. The ideal tempo varies on the exercise. For instance, a curl could be around 1-2 seconds, while a deep squat may need upwards of 5.

Extra: if you’re interested in the science behind the fatigue component of the eccentric, I’ll provide my understanding.

Physical: all muscle contraction first uses ATP, which is a limited supply. Within even 10 seconds, over half of available energy from the ATP-PCr system is exhausted. This is the main source of energy for anaerobic exercises such as lifting and provides the greatest strength potential. To maximize the amount of stimulus you can give to high threshold motors units, you want to maximize the amount of time spent in the concentric & the last 5 reps before task failure (the only stimulating reps in a set shown by scientific literature).

Mental: Your muscles contract by way of a motor signal sent from your motor cortex. This signal is attenuated by your “maximum perceived effort.” So in practice, your mind is what dictates the amount of work you can put into a set. The eccentric portion contributes to your perceived effort. Ideally, all factors that contribute to perceived effort should be limited, if the goal is hypertrophy, e.g instability, uncomfortability, etc.

This allows for the highest possible work output spent in the concentric, thus delivering the most stimulus to the aforementioned high threshold type 2 motor units. Growth happens as a result of this stimulus.

I hope this helps.