Your mage is not much more dangerous than your average infantryman
Fighter jets, helicopters, warships, and submarines are all speced to take the occasional lightning strike and be fine. These are common enough hazards that being able to take it is part of the design.
Most drones will probably be fine too since that they are not grounded; so, not a whole lot of potential for destructive currents to be drawn through it.
Tanks, APCs, and humvees are the only vehicles that have anything to worry about. There have been reports of lightning hitting tanks and other ground vehicles. Normally it will fry a bunch of electronics, but wont harm the people inside or completely destroy or disable the vehicle. So while a bolt of lighting will do more damage than a standard infantry riffle, there are plenty of man portable tank killer weapons out there that that are arguably more effective than lighting.
Good thing about lightning is that is can spread out and hit several infantry at once... but so can Machineguns and RPG, so still not really proving any more dangerous than many infantry weapon systems.
All this said, there is one factor that REALLY limits mages in modern warfare and that is human perception. The modern battlefield is full of all sorts of kill-you-over-the-horizon weapon systems that a fantasy mage would have no counter against. Even if her magic were powerful enough to be a real threat, all of her fancy light shows would attract a lot of attention and she may just find herself exploding into a mist when she finds herself on the receiving end of a smart shell fired from 10km away.
Ways to make her much more dangerous
Strong control over directionality
Once released, the lightning acts like regular lightning only directed at a target.
All this said, HOW directed at your target could matter a lot. Several countries have experimented with electro lasers for shooting down planes and missiles using far less power than an actual lightning bolt. Lighting by its very nature spreads out a lot and wastes a lot of its energy, most lightning bolts never even reach the ground in any apparent way, but if all the power of a lightning bolt were actually directed into a single point, it would be incredibly destructive. A typical bolt of lightning is about 1,210,000 kilowatts, but electro-lasers in the 30-120 kilowatt range are sufficient for blasting a small hole in an light armored vehicle; so, if your mage were to actually direct the full power of a lighting bolt into a target, it could easily take out a main battle tank or cause significant damage to a warship.
Magical answers to modern information driven warfare systems
A mage that relies on thier own senses can't attack or defend against modern information driven warfare systems no matter how hard she hits. If the mage starts to cause to much trouble, she will quickly find herself on the receiving end of a super-sonic artillery shell or missile fired from 10s or even 100s of kilometers away. If modern tech can attack her from outside of her nexus of awareness, then no amount of magic will save her.
That said, a mage may have tactically useful powers other than lightning bolts that are comparable to or otherwise counteract modern information warfare systems. Some examples include:
- Recon Powers: If she can command familiars, astral project, and/or scry, then she can gather situational awareness over a whole battlefield in ways that are comparable to modern methods. So while she may still be vulnerable, she at least has the ability to try to hit first.
- Cloaking Powers: If she can bend light to make herself invisible or hide herself in darkness or hide her body heat in mist or something like that, then she may not be able to see over-the-horizon threats, but she might at least be able to minimize the threat that they can pose to her.
- Defensive Powers: If she has some kind of protective charms that passively protect her, or perhaps has some kind of precognition or danger sense so that she knows exactly when, where, and how to defend against attacks from distant weapon systems, then she could use magic to stop attacks that she would otherwise not even be aware of.
Make her an assassin, not a solider
The ability to summon lightning is a great assassination talent since she does not need to carry any weapons to be able to kill a person of interest. She could get on a plane, walk right into some enemy dictator's political rally, and cut the head right off the snake after walking right through numerous checkpoints.
Allow her to attack from the sky
The power's usefulness could also be improved if she could summon lightning from the sky, instead of shooting it from some part of her own body, because that would make her attacks virtually untraceable. Modern warfare significantly favors the combatant that can stay unnoticed until the shooting starts, but being able to stay unnoticed AFTER the shooting starts is the holy grail of modern combat systems. In this case, she could kill political leaders or attack enemy positions from a near-by vantage point, and no one would know where to even aim to defend themselves. Coupled with strong control over directionality, she could wipe out entire military convoys or sink fleets and modern militaries would have a very hard time knowing what to do about it.