I have two grant applications I need to do very soon. One is an internal grant for summer salary, the other is to be director of an NEH Summer Seminar.
Grant applications are particularly difficult: the most difficult kind of academic writing there is.
1. It is harder to explain why your project is significant, why it deserves funding, to non-specialists, than to actually do the project. More hangs on audience reaction (yes or no on the grant). Percentages are low.
2. Grant writing depends on other people aside from yourself to get things done. You need to plan for other people to do certain things, like write letters of recommendation or give institutional approval. There are two potential problems: not asking people in time, and having to wait for people to do things before you can do other things.
3. There may be other skills involved, of the non-scholarly variety, like doing budgets or understanding bureaucratic red tape.
4. Deadlines are more hard and fast. For your own projects deadlines can be self-imposed.
#4 is actually an advantage, since a hard and fast deadline is easier to keep. You can always change your self-imposed deadlines, and most changes are postponements rather than anteponements.
For people who are actually better at coordinating things with other people and arranging logistical details than at writing scholarship, 2 and 3 might not be a problem. For me they are, since I am considerably worse at those than I am at doing the actual work. I am very bad at budgets, for example.
I can craft a convincing narrative, so # 1 is not going to be a problem.
Another problem with grant writing is that you can spend almost all of your research time asking for time and money to get your research done. If your time is very limited in the first place, it might not be worth it. You might be better off just doing it. For me it is worth it, because my research time is not all that limited, and I've published a lot recently so a grant does more for me than another article.