That's not a gernade that's their poop after all they don't call em swooping/pooping pigeons for nothing.
You realize you'd just die to his pistol right? No. Experienced people just ram a vehicle into another to stop it and use tacticals with meltaguns and meltabombs. That is much muuuuch more efficient and viable.
One of the smaller Eldar Guilds have made this their core gameplay. Hawks and Dragons in a Serpent, Hawks jump out and circle, keep the enemy vehicle in place and to pick off anyone who jumps out, Dragons melt the vehicle. Beautiful to watch. They just let the PuG do the dirty work of taking points. Nah, Eldar are the teamwork faction remember, there'd be a couple of DAs to melt his face for him before you'd go down.
I also know of eldar guilds that abuse the physics of the ws and falcon to ram rhinos into the air or block the. Till they can't move and in the chaos just FD it to death and if you try to get out you get killed by lawn mowers, if you try to manovoer out then the faster WS or falcon move a faster then you and block you out all the while Hawks poop on you.
Not any more than the AP Hawksear grenades, though they require some practice to properly land. Both require the Hawk to get relatively close to the enemy while being completely exposed and visible on the radar. Risky, but they are rewarded greatly. Ramming an enemy vehicle may work if they just keep rotating around the same route or the map is particularly cramped. But on wider areas like Medusa and Torias, this method becomes far from reliable as meltas and meltabombs have shit range and can be evaded at the pace of backwards acceleration. The Gravcannon + Multimelta combo only requires the AV-team to get relatively close to the enemy vehicle, with the own transport needing only to serve as a short roadbump to slow the enemy vehicle, after which the snare prevents it from escaping while the Multimeltas rip it apart in two bursts.
Wew what a difference, totally worth carrying a grav weapon for that small advantage on that specific situation that doesn't even occurs 95% of the time since transports are usually just parked somewhere anyway. I keep telling myself when I play other factions "oh gee wee I wish I could use a grav weapon right now to do that specific thing instead of racking 40-50 kills per game with this autocannon/ace dakka or instantly kill any vehicle with that melta bombs bundle " I can't believe people are still trying to argue grav weapons are great, it's just silly.
Here is the thing. While I actually like grav guns and find them enjoyable to use (although I almost never use them), when I see them against me, my first though is "huh, free kill", because I know that I can kill them before they can kill me. They can be good in a group and multiples of them can be used to kill vehicles (good luck getting multiple people on your team to use them) and still have use against infantry, but even for slowing vehicles down, they can still outdrive the grav gun, so unless they are almost dead already, it will still get away.