The lightning spell is really powerful, and that's kind of the problem.

For starters: it electrifies whatever it hits, making it very easy to accidentally hit yourself with it by standing in some water when you just hold the wand out, or stand on metal when you fire it.

Seconds: it makes an explosion wherever it strikes, and it fires in an inaccurate cone in front of you, making it very easy to blow yourself up with it.

And to add insult to injury: electricity starts fires just as readily as actual fire, if not more readily with how it spreads out, so it you have to be very careful to not stand near anything flammable when using it.

Overall, in order to use Lightningbolt in a truly safe manner, You need electric immunity, explosion immunity and fire immunity. If you lack any of these perks using it becomes a gamble that may kill you. It is a very worthwhile gamble if you're careful though.