Nickel gives you a lot of creative options if you have the control over what's happening - as it can become so different things.
So I guess that could be an backhanded way of of saying the resulting look of the glaze can be unreliable depending on the chemistry.
This is nickel silicate (Garnierite). Nickel sulfites, chlorides and fluorides are all various shades of green.
Excess Nickel crystallizes out of a glaze as it melts., but it's far more soluble in glaze than chrome which crystallizes out as ugly brown dots..
So if you add an excess, the cooling speed is going to greatly affect the look of the glaze.
This is a "glaze" I call Olive Speckle.
100 Ferro Frit 5301
6.3 Nickel Oxide
Seen here are four different guises.
1.) This is fired to Cone 06 - Most likely undissolved green Nickel Carbonate which was the starting ingredient in this case, or Nickel Silicate crystals over a brown nickel glaze.
2.) This is the same fired to Cone 04 in a bisque firing, so there's more carbon monoxide around and it cooled more slowly. More nickel has remained in the glaze resulting in a deeper yellow brown.
3. & 4.) And finally, these are the same "glaze" with an extra 10 grams of Silica - fired to Cone 6 with a slow-cool and a fast cool, showing various combinations of nickel. Both are likely Nickel Fluoride using fluoride from the frit. Notice the bubbles and the pinholes not annealed in the fast-cooled tile on the right.