Isolating varnish

I will assume that you are talking about the use of isolation varnishes between layers of oil paint. Isolation varnishes between disparate paint systems are a different idea and may or may not be advisable (eg applying a coating over a very absorbent aqueous paint system to cut the absorbency before applying oil glazes) Also, the concept is different with some other mediums. I can see no really issue with using clear interlayers of acrylic dispersion mediums between layers of acrylic paint. Acrylic dispersion are an excellent adhesive, where drying oils really are not.
This use of isolation varnishes in oil painting should be discouraged for a number of reasons. First, it is generally a bad idea to reduce the mechanical tooth of a paint layer, which could promote delamination or flaking of the superimposed layer.
Second, adding the varnish layer between paint layers will introduce an unnecessary solubility issue. Even if it is covered by additional oil layers, the varnish could be attacked and undercut during a restoration campaign resulting in the loss of all subsequent layers. For instance, a layer of natural resin between paint layers will create a paint stratigraphy that is sensitive to hydrocarbon solvents, even those containing a low proportion of aromatics. A layer of shellac between oil paint layers introduces a sensitivity to alcohols, etc.
Additionally, the use of varnish interlayers creates a more complicated paint stratigraphy. We know from examination of historical paintings that the more complicated the stratigraphy, the more likely there will be some failure in the future. This does not mean that one has to create paintings in only a few layers, but you should aim to use as few layers as is necessary to create the desired effect.
The varnish interlayer will also respond in a different manner to movement of the substrate than will the paint layers below and above it. It will also age differently. The flexibility of the varnish may change drastically over time making it less flexible than the layers that it is covering. Etc, etc. So, for the above reasons, and likely many that I am not thinking of at the moment, it is really best to avoid the use of isolating varnishes in oil painting.