Most people cannot fathom the idea of meditation on the Absolute, which is formless and attributeless. Only a fortunate few are able to attain that highest state of realization. The path of bhakti (devotion) is considered superior for those who are unable to realize the pure Self
Those who contemplate on the non-manifest Self beyond all forms think that saguna worship (the worship of form) is inferior to nirvana contemplation. Nirguna contemplation is focused on the absolute truth, which has no form, no name, and therefore no attributes. Although one cannot worship the formless, it can be a subject of contemplation. Those who find delight in the contemplation on the formless, nameless, and attributeless Brahman think that the worship of God with form and name, with attributes, is inferior. The path of nirguna is difficult, and there are chances for the aspirant to slip.
Ordinarily the mind needs a concrete object upon which to focus and rest, for the mind is in the habit of depending on concrete objects. It requires great preparation for the finest faculty of mind, buddhi, to be able to understand the difference between the Self and the non-self, the real and the unreal. But when buddhi is sharpened, one has the capacity to go beyond the realm of forms.
In this video lecture Swami Rama speaks extensively on meditation on the formless: