Scarabs move an initial 6" in the movement phase and move max 18" during or after the shooting phase. Hellions move 12" in the movement phase and only max 12" during and after the shooting phase. Thus, with scarabs you are more capable of moving conservatively, waiting to see how the dice go in the shooting phase, and then make a decision based on that. Hellions have to come within 12" of the enemy in the movement phase (preferably within 7"). Thus, even with a good run roll you might not be able to get out of your opponent's charge range and are more exposed then the scarabs.
Jump infantry has a certain advantage over beasts. They don't roll for difficult terrain, always moving the full 12". In addition, they can move 18" without charging an enemy, whilst beasts need that charge move to gain extra movement. Thus, early on into the game jump infantry are more mobile.
I think this brings up two different kinds of playstyles according to the unit type. Beasts are almost always combat orientated, jump infantry don't need to be. Beasts are also more about waiting, calculating and then pouncing. Jump Infantry, on the other hand, can be in your face earlier. They are faster but also more risky. Once you go for the charge, there's no going back. However, there is also a lot less your opponent can do hamper your movement.
Taking into account unit types can also help you play against your opponent's list. I've played against thunderwolves all the time. One of the things to remember is to NEVER give them the charge. As long as they aren't charging, they are slow. This gives you the advantage. If you let those TWC charge into your face, their threat range will ensure that there's no getting away anymore.
On the internet, we often see people just talking about threat ranges, but that's only half of it. How do you get there? In which phases is the majority of that movement? There are many other factors that are fundamentally gonna effect how they perform on the battlefield.