It's actually a lot harder. The fly-bys invovled gravitational slingshots, which not just steered the probes but vastly accelerated them. Without the slingshots, human rocketry of the time would not have had the capacity to get probes to solar system escape velocity.
Our understanding of asteroids is now reasonably good. We know they come in solid and rubble-pile formats. We know that several show signs of lava flows, suggesting they come from disintegrated planetoids with a liquid core. We've managed to land on some and even retrieve samples from one.
Our understanding of comets is much weaker. We know that several appear to be multiple bodies fused together with ice (the one we tried sending a lander to was of this type) and the break-up pattern for Shoemaker-Levy 9 is suggestive (but not proof) of it having been a composite structure too. The one icy body we've seen in the Kuipier belt was also two bodies frozen together. Photographs of the core of Halley's Comet also suggests two bodies fused together with ices. This leads to some interesting questions. Obviously the probability of this configuration is very high, but what is the probability of other configurations? Is it so incredibly low that the odds of finding single-body or triple-body configurations is practically non-existent?
(This is kinda important if NASA is serious about a DART-type project to redirect one.)
Our understanding of solar system captures is non-existent. If there are extra-solar objects flying through every few years, then there is a definite non-zero probability that there are significant extra-solar objects that have been captured by the solar system in the last 5.5 billion years. That would be kinda very useful to know.
Current solar system formation models don't work well, but planets have sorted their composition by mass. Studying them only tells us so much. Asteroids are more useful but hit each other too often, so we don't know how much mixing and reconstitutuing has occurred there. The Kuipier Belt is much better, from that point of view. The density is far too low and most of the objects far too small. A lander could obtain a lot of extremely useful data about how the solar system formed that we can't obtain closer.
The Oort cloud is theoretical only, at this point. The only way to find if it is real is to send something out there.
If we're to get any probes in the future to go between solar systems (and such missions are entirely possible), we need to know a great deal more about the heliopause and the galactic winds.