Nikolai Myaskovsky is doubtless one of the best composers that even most college music majors never heard of. He lived from 1881 to 1950 and never became as famous as some of his contemporaries, such as Prokofiev and Shostakovich. However, he is noted as "the father of the Soviet symphony," and he wrote 27 of them, an unusually large number for a 20th century composer.
Given his large output, it is understandable that his symphonies are not of similar quality. Some are very good, some are no better than mediocre, if that.
Very often symphonic composers will fall down on the slow movement. The simple fact is that it is considerably more difficult to keep slower music interesting than it is with faster pieces. A long list of fairly boring slow movements in symphonies could be mentioned, although there are some notable exceptions, including the heart-rending "Going Home" movement in Dvorak's "New World" Symphony.
Here is a LINK to the second movement of Myaskovsky's Symphony #20. In my humble opinion, it ranks with the great slow movements in symphonic literature. It is less than nine minutes long, and well worth the listening.