"We've been emped" only applies to when the enemy detonates a big emp. Not the grenade. Not too sure how grenade works, never tried it. I will now thou.

The EMP grenade is good for knocking out equipment like sentry guns and recon drones and will temporarily disable the radar of a person within the blast radius.

The EMP in the Support Strike Package, on the other hand, disables electronics and air support of an entire team. That's when you see a big flash and hear 'We've been EMP'd.'

If anything, you should get 3-4 because it's not like it's meant to really help kill players, much like the Smoke Grenade, but only effective against equipment. Even against players, the blast radius is ridiculously small compared to the Flashbang (where you can be 100 feet away and turned away, yet still get the full brunt of it) and even the Concussion Grenade, and its detonation is VERY fast, so it's not like you can get a whole lot of distance on tossing it. Players essentially have to be right next to the detonation to be affected by it.

I also believe it does work on the Assault Drone, but you actually have to directly hit it and not simply have it in the blast radius, so it's tricky to do.

Also, to know if it hit, just listen and watch for a hitmarker and you'll know someone/something was hit by it. The problem is is that the effects merely last for a few seconds, about as long as the Concussion Grenade, IIRC, but it's not like everyone who is affected by an EMP effect is instantly helpless just because of a fuzzy screen, loss of mini-map, and possibly electronic sites, so you shouldn't toss one into a building and expect to rush easily like if you used a Flashbang. Just like how you shouldn't expect an easy win over every non-Assassin opponent after using a normal EMP.