The Worship of The Formless – Bhagavad Gita Chapter12 Verse5

क्लेशोऽधिकतरस्तेषामव्यक्तासक्तचेतसाम् ||
अव्यक्ता हि गतिर्दु:खं देहवद्भिरवाप्यते || 5||

klesho adhikajara stesaamavyaktaasakta chetasaam
avyaktaa he gatirdoohkham dehabadbhir baapyate
(Bhagavad Gita Chapter12 Verse5)

The worship and meditation on the formless aspect of God by the people who are attached to the physical body and the mind is very difficult, strenuous and too much hard work. Because, the people who identify themselves with the physical body and the mind, find it troublesome to conceive and understand the formless spirit.