Electric Harps-
The thing that defines an "electric harp" is the presence of a pickup on each and every string of the harp.
If you have looked into amplifying your harp, you have seen systems with one or more transducers mounted inside the soundbox of the harp. Maybe there is a little microphone suspended in there as well. As the saying goes, results will vary. When you collect vibrations from inside the soundbox, you are doing exactly that, collecting the sound inside the soundbox. But is this the sound you would really like your audience to hear?
A potential problem with internal transducers is that you will have a resonance peak, that is, particular notes that jump out louder than the rest. This is the natural resonating point of your harp's soundbox. Also, depending on exactly where you place the transducer inside the harp, you may have lots of bass but weak treble, good melody but weak bass, etc. Worst of all, you are susceptible to feedback, where the soundbox vibrates in response to the soundwaves coming from your loudspeakers, and this gets amplified, making a rapidly escalating roar that is very unpleasant.
It's the right tool for the job-
Placing a pickup against each string produces an even response and avoids the above problems. Pickups designed for harps should be sensitive on the top surface, where the string is in contact, but less sensitive on the bottom, or mounting surface, to capture less of the energy of the soundbox. In this way, the activity of the strings is captured, but not other sounds.
A lot of the interesting sound happening when you play a normal acoustic harp is never heard, since the surface area of each string is so tiny. Our ears cannot detect the weak direct radiations of the strings. We rely (in an acoustic harp) on the amplifying effect of the soundboard and soundbox to make the harp audible. But this filters the vibrations, sometimes in a good way, sometimes not. The strings are in intimate contact with all parts of the harp, and respond instantly to the entire spectrum of sound present. You might hear the subtleties with your ear next to the harp, but your audience usually doesn't. By amplifying each string your audience can hear everything you are doing, clearly and cleanly.
A solid body harp, like a solid body guitar, can be amplified as much as you desire, up to the limits of your electronic gear (amplifier, loudspeakers, effects boxes, etc.). However, most harpists want play unamplified at times. An acoustic/electric harp with pickups on every string will sound richer and more interesting than a simple plank-bodied electric harp, and will be more powerful and reliable than an acoustic harp with transducers inside.
Acoustic/Electric harps
Irish 34 string harp, with pickups
Electric Harp, solid body