NGOs are long-term functioning organizations that usually run on donations and have broad objectives. The idea here is to form short-term NGOs that have more specific objectives. As a simplistic example, a regular NGO works to maintain the cleanliness and portability of water in the lakes in a state. Against, a short-term NGO will strive to clean one lake and only once. After the job is done, the NGO will be dissolved.
There are 4 major challenges that regular (long-term) NGOs face. First, lack of funding. Short-term NGOs will be initiated once the asked funding is acquired, thereby solving the problem. Since they have a niche objective and will not be working for long, the funds required will be comparatively smaller, thereby increasing the chances of acquiring them.
Secondly, lack of strategic planning. Again, in the case of short-term NGOs, the plan will be ready and approved by authorities before starting the work.
Thirdly, regular NGOs compete with other NGOs to maintain a regular flow of funds. Where there could be collaboration, competition leads to NGOs working in isolation. Short-term NGOs would not rely on long-term networking for their survival.
Lastly, lack of maintenance. This issue won't arise in the case of short-term NGOs.
NGOs depend a lot on volunteers - for actual work and also for planning and consultancy. A steady flow of volunteers is not guaranteed. Since it is not a corporate, knowledge transfer rarely happens systematically when newer personnel join. Short-term NGOs will minimize this issue.
Volunteer motivation and interest decrease with time. Short-term work could be done with sustained motivation.
There are several small (comparatively) issues that arise that need professional help, for which short-term NGOs are well suited.
An example of such an isolated task is to "create a policy that disallows construction on a specific area that is important from an ecological point of view". This will require experts in environmental law, ecology enthusiasts, conservationists, writers, and media support. An NGO could be formed for that very purpose. After the government creates a policy in favor of the NGO, the NGO could be dissolved and the people are free to pursue other projects.