My two cents:
Hydras do a metric ton of disturbance if you actually land one on someone, and right now they have the potential of doing a LOT of damage if multiple rockets hit irregardless of screwing someone over if just one of them tags them off the track, so I think their potential is balanced by how hard they are to land on people. Saying that you shouldn't be able to just reset out of the spinning effect though (this goes for most weapons, there should probably be a delay where you can't reset x seconds after being hit by something). Making them capable of clearing mines might be something to consider, or maybe upping the accuracy of the homing. (multiplayer latency is also a problem because I've noticed the hydra has to hit on the victim's end, not yours, for it to work. This goes for all weapons, though).
Mines are dangerous, but I feel they do need more utility because they're easy to dodge with how gimped they are with the cars' launch ability and at race speeds there's every chance that hitting a mine does nothing but scrub maybe 100km/h off of your speed. I agree that you shouldn't be able to put them anywhere and expect to get a hit, but that's the nature of multiplayer; people are smart-enough to find paths around them if they're placed in places that allow it, so their strength is in *forcing* them to do so in a way that substantially slows them down, or placing them in a position where hitting them is unavoidable.
Unfortunately as said, as of now it's easy to dodge most mines by just initiating a short (quarter-second preload) launch to hop over them, so it's clear that the mines' hit detection needs adjusting to make them more of a consistent threat. I shouldn't be able to dodge a mine by hopping over the ground it covers whilst my tires physically pass through the red trigger; I should need more of a jump to clear them.
If Rob has idea for buffing mines, let's let him do what he wants and see what happens. Historically though, area-denial weapons are often more effective than their 'hit rate' lets on, because their effectiveness is as much in being hit as being a deterrent (Mines and Bolts in Blur historically have low hit ratios because of this fact, that doesn't mean that they're bad.)
Still, let's wait and see.