Disadvantage: Of course the major (and only) disadvantage is less daylight time for eastern states. But let's look at what it actually means in terms of social and economic drawbacks -
Productivity: Even though it's rich in minerals (iron, coal, mica, limestone, manganese), oil and tea, they are still economically left behind.Higher energy consumption: More usage of electricity in offices as the sun sets around 3/4 pm. A conservative estimate shows that starting the day an hour earlier would result in a saving of about 550 MW of power in northeast India alone.
Social stigma with night / darkness: Every city has an apparent safe time limit for strolling in the night. Like for the west, it might be around 10 PM. This would be earlier for eastern states by at least a couple of hours. This in turn might cause lesser customers at the night life (restaurants, pubs) venues, and hurting the economy a bit.