What are the disadvantages of myelin
Many invertebrates possess myelin. It is a misconception that invertebrates lack myelin.

The world speed record for a traveling bioelectric signal is held by the myelinated axons in the abdomen of the Penaeus shrimp!

Please take a look at this website or this review to find a highly recommended comprehensive website about invertebrate myelin from about a decade ago. Here you will find a list of references and additional readings if you are so inclined.

As an additional note: invertebrates also possess and very often make use of ensheathing glial cells, such as is the case with sensory neurons in lepidopterans (moths and butterflies) as depicted below. I've labeled the sheathing cells in pink and red. Please understand that it is currently still unclear whether these cells secrete myelin, though they are thought in some way to insulate the neuron. What you are looking at is a hair-shaped sensillum that is innervated by one or several sensory neurons; these are the basic functional units for sensing movement, proprioception, smell and taste and they exist across all invertebrates, often lining the cuticle and across the entire body, e.g. on legs, on the antennae, in genital tracts, etc. They are best studied in hexapod insects.