While a Roof-top restaurant can be quite cool from a customer's perspective, if executed well, there are some challenges for the restaurant business owner.
1) In most states in India, you can't get a trade license for a pure roof-top restaurant. The rules state that you cannot have a kitchen in a roof-top/terrace. So to comply, your kitchen will need to be in one of the lower/properly built-up spaces in the building. In practice though, most roof-top restaurants seem to have some part of their kitchen in the terrace floor - e.g. Tandoor for making Rotis, or even the entire kitchen on the terrace floor housed in an unapproved constructed space on the terrace.
2) If the kitchen is in a different floor, transporting the food through stair-cases becomes an issue. So you will need to install a service lift (may be tough if the building structure does not allow it)or have a wide dedicated passage for the food to be carried through. While the issue will be a little lesser in the case of a buffet restaurant or a fine-dining restaurant (where the service levels can be a little bit more relaxed)
3) Creating a temporary roof for the terrace area is not easy, especially if aesthetics need to be taken into account. Barbeque Nation is a great example of a great temporary roof set-up - Their tent-like roof structure looks nice, withstands weather conditions. Rough cost estimates for these structures is around 2-3 lakhs per tent.
4) In most buildings, the floor area of the terrace is not something you can straight-away use for a restaurant. So you may have to spend reasonable effort and money in civil works like flooring etc. Civil works in general are expensive and time consuming.
On the positive side, the biggest benefit you would get is the lower rental in square foot terms. In places which have reasonably good weather (like Lucknow), customers also enjoy a nice terrace restaurant ambience and this may help improve customer foot-falls if executed well.