When talking about the bsnl I think as a user of a bsnl it suffers from various problems like Very limited coverage , low speed and shut down of network for a long time.
So the demerits that I have found in bsnl.are as follows-
Poor network coverage- As a reguler user I can say that bsnl have a better network in cities but when you go to the outskirts of city it becomes weak and u will not be able to use your voice and data.
Low speed problems- It only doesn’t have a weak network it also have problems related to low internet speed when you go inside a room it is possible that network become weak and a decrese in internet speed.
If any problem occur in the network like a fault on your nearby mobile tower it the company will take a long time to fix it.
Hope it will help you.