I use the Berg copper and blue toners all the time for a few reasons:
1. they really are metallic toners, not dyes.
2. they can be reversed with paper developer, although contrast is lessened.
This re-developing method also is effective for split-toning.
3. they are liquids, so you do not risk the health hazards involved in mixing
powdered chemicals. (The Museum School in Boston, where I teach, will
not even allow me to use powdered chemicals, despite having just installed a super
ventilator over the sink.) At home, I do use a mask and mix powders,
but I try to avoid it.
4. they can be used on b & w photos, as well as, van dyke brown prints.
5. they are gorgeous when used with each other on the same print.
I do not like to use any toners on r.c. paper, which is a type of surface that
suffers when kept in liquids too long.
(author, New Dimensions in Photo Processes)