Depends on four things really...
1. The ship you are flying and the amount of engineering you have. Ships in ED are (for average players?) very unbalanced. Get yourself into the big ships and you already have an advantage, moreso if you have engineered weapons / shields.
2. The rank and ship of the NPC. It makes a difference, and a high ranked large ship can be a challenge. As above though, less so if you are also in a large ship, and if you've engineered it, the advantage is back with the player.
3. The environment. If you are fighting an NPC along with a bunch of other NPCs, be they system security or bounty hunters, then that is much less of a challenge than if you are one on one and the NPC can concentrate it's fire on just you. So typically, at least in my experience, any location such as a RES or a Nav Beacon, and to some extent even some CZ's are much less difficult than an encounter with a pirate lord where you are on your own. Of course, CZ's where you are outnumbered are the polar opposite...
4. Pilot skill. That said, NPCs are designed for all of us, hence I guess some of the complaints that emerge on the forums about them being too easy, but I suspect for the average player they can certainly provide a challenge. After all there are a wide range of NPCs, and a fairly wide range of ways of interacting with them, from really easy to pretty challenging. For (I would imagine) most players, if they want a real challenge they just need to pick the right one.
1. The ship you are flying and the amount of engineering you have. Ships in ED are (for average players?) very unbalanced. Get yourself into the big ships and you already have an advantage, moreso if you have engineered weapons / shields.
2. The rank and ship of the NPC. It makes a difference, and a high ranked large ship can be a challenge. As above though, less so if you are also in a large ship, and if you've engineered it, the advantage is back with the player.
3. The environment. If you are fighting an NPC along with a bunch of other NPCs, be they system security or bounty hunters, then that is much less of a challenge than if you are one on one and the NPC can concentrate it's fire on just you. So typically, at least in my experience, any location such as a RES or a Nav Beacon, and to some extent even some CZ's are much less difficult than an encounter with a pirate lord where you are on your own. Of course, CZ's where you are outnumbered are the polar opposite...
4. Pilot skill. That said, NPCs are designed for all of us, hence I guess some of the complaints that emerge on the forums about them being too easy, but I suspect for the average player they can certainly provide a challenge. After all there are a wide range of NPCs, and a fairly wide range of ways of interacting with them, from really easy to pretty challenging. For (I would imagine) most players, if they want a real challenge they just need to pick the right one.