Are there any disadvantages of using a 20 bit address line in 8086 microprocessor?

There are two primary disadvantages to a 20 bit address.

20 bits, i.e. 1MB, is not a lot of memory, and you can not, today, do a lot of work with it. In fact, the original DOS memory model only provided 640KB of addressible space, of which around 500KB to 600KB, depending on configuration, was truly user space.

The biggest disadvantage is that, with the segmented architecture of the 8086/8088, you can only address 64KB of data with one instruction. This is due to the 16 bit architecture, and due to the fact that you had to reload a segment register (a second instruction) in order to address any arbitrary byte in memory.