There is far less knockback with the Halo 5 weapons. A direct rocket hit to an unmoving Mongoose barely upsets it, where in past games it would be sent flying across the map. Lowering the player gravity no longer affects them while driving.
It's not in the best interest to kill an enemy when circumstances prevent them from scoring, or give you a strategic upper hand. For example in Grifball you NEVER kill an enemy player who is downfield from your Runner. Ever! Letting him kill you instead prevents him from spawning in front of your Runner, which may prevent your team from scoring. You also don't kill a player when you are 10 seconds away from grabbing a Flag in CTF, for the same reason. So many people do not get this. In a RR game (where the map does not kill the player) if you see an enemy who is unable to reach their vehicle in a timely fashion you don't splatter them, you instead leave alone and score some hills while they sort themselves out. It makes no sense to kill them (but most do anyway because it's fun).
I believe this version of Rocket Race, where it is nearly impossible to kill a player anyway, works best with a 10 second pulse kill zone, or a 10 second sweeping kill zone (like i use) Players can choose to run for a hill but they will more than likely be killed by the map prior to reaching their destination. It makes falling out of an upset vehicle a very risky situation, and running back to mount your vehicle before the kill zone hits adds to the tension of the game, rather than frustrating you with slow movement speed.
There are areas of maps where players may lose their vehicles, of be in a situation where it takes a long time to retrieve them (cliff & Mancanon on Deadlock, Clif, central pit, base top platform on Avalanche). In these circumstances you need to provide players with a swift method to return to the game without frustrating them further.
Third point, and this a point i make to any community developing new game types. You should try to punish players who are exploiting your game, but NEVER at the expense of players who are trying to play by the rules. Say we are playing a friendly game of RR (where no one is trying to exploit the hitting hills off their vehicle) and i accidentally flip my Hog, and land in a Mancanon which flings me to the other side of the map. Why should i be punished with excruciatingly slow movement speed to get back to my vehicle so i can continue to play? Solutions to greifing should never punish regular players, and the mechanism to fight greifing should therefore be invisible to regular players.
Timed/sweeping kill zones are not ideal but they are all we have until 343i provides us with the tools to force players back into vehicles. They help to prevent griefing, while also assisting players who are unable to reach their overturned vehicle in a timely manner.