[Wavytone]

Kellners: very limited. Basically an obsolete design best suited to f/15 refractors. 40 degree field of view. These days they're junk, and not made anymore.
If you want a 3-element eyepiece, buy a set of Edmund RKE's, they are as good as it gets with 3 elements.
The only other 3 element design on the market are the TMB supermonos, excellent planetary eyepieces (high contrast and sharp) but limited field of view.

Erfle: another very old design, better than Kellners; they give nice wide low-power field views in an f/8 scope, but no good in faster scopes, and no good at low focal lengths because the eye relief is very very short. So short that Erfles were never made much below 20mm focal length, and even at that size an orthoscopic was usually preferred. For a while I had a monster army-surplus 60mm Erfle, it needed a custom barrel as it was bigger than a 2" focusser, and the field of view was monstrous, BUT to be honest my 40mm TMB Paragon is a lot better in an f/7 refractor.

Most modern wideangle eyepieces will do a lot better than an Erfle - even the cheap GSO's, Orion Stratus, Baader Hyperion, Vixen LVW, TMB Paragon, TV Panoptic, Radian etc.