The use of gravity assist has been an integral part of space exploration. Gravitational slingshots have been used time and time again to send spacecraft to areas that would be impossible to get to otherwise by providing the spacecraft with increased speed. Accordingly, spacecrafts are able to get places faster and use less fuel.
(Voyager aircraft performing gravitational slingshots)
Gravity assist, does, though, have its limitations. First and foremost, relying on the use of a gravitational slingshot means that the mission has to take place in a relatively small window of time. For instance, if we wanted to perform a slingshot around Jupiter in order to get to Pluto, it would make sense to time the mission so that all three planets were closest to one another. This means that NASA (or whoever else is launching the probe) has to keep to their schedule very tightly or risk having to potentially wait years.
Furthermore, there are limits to how much speed a spacecraft can obtain through performing gravitational slingshots in our Solar System. The most speed a spacecraft can gain through one gravitational assist is the speed at which the planet orbits the Sun. Thus, the most that Jupiter can be used to accelerate a spacecraft with one gravitational assist is about 13 km/s. Though this is extremely fast, it is still not nearly fast enough to make interstellar travel feasible. If we wanted to perform multiple gravitational slingshots to further accelerate a spacecraft , we would need to develop more fuel efficient travel methods since performing multiple gravitational slingshots would take too much fuel.
