What can we call " an employee who is under-productive but the quality of his work is enviable"

A pleasant expression for an employee who has remained under-productive despite several feedback.

(QUANTITATIVELY WORST)

There are workers who are unable to churn up BIG numbers but the quality of their work is enviable/unmatched by peers.

(QUALITATIVELY BEST)

• A noun, an adjective or matching idioms.

A person showing "momentary sparks" / "frequent flashes".

The person can also be referred to as "a gold nail" [my coinage]. It means you can neither throw it away as it is made of gold, nor you can use it as a nail, because it is valuable.

Artisan may apply to a person who focuses on quality over quantity:

A worker in a skilled trade, especially one that involves making things by hand:

ODO

Someone who slowly produces high-quality work is said to be painstaking.

From the ODO:

Done with or employing great care and thoroughness; painstaking attention to detail - 'he is a gentle, painstaking man'.