I'm going through Kent Kennan's book on 18th century counterpoint, and early on he touches upon some finer points concering agreeable or not-so-agreeable chord progressions. Only a handful of progressions, I gather, are considered "bad" — so weak as to be entirely avoided, perhaps:
Note: there is no distinction made by Kennan between major and minor here, I think (?), major is only suggested for the sake of convenience.
V to IV, both chords in root progression — because this progression is "generally heard as a reversal of normal harmonic progression" (p. 24, 3rd edition) — alright, fine.
ii to I, also when both are in root position. No reason explicitly given, for why this is weak. "The ii should either progress to a V before going to I or move to a I 6/4 if a cadence is involved" (p. 24). I suppose this may be a variant of the "V - IV" rule. Can this be confirmed? Anything more to say on the matter, besides the notion of a "reversal" of normal harmonic evolution?
The first inversions of the vi and iii chords (vi6 and iii6): to be avoided unless one uses them by way of a stepwise bass: "Employed in that way [with the bass passing stepwise], they do not actually express the degree function of vi or iii but are what might be called "contrapuntal chords.” (p. 24)
Not an explicit reason provided for the third rule, either. And this is the more intriguing one. Are there any underlying principles that can be elucidated here, beyond what were the conventions established by Bach?
I would guess that this has something do with the vi6 chord and the iii6 chords being so similar to root position I and V chords, respectively, sounding like embellishments of them. But it seems like there's something more to be said here. Is the "stepwise bass"-exception tied to more specific voice leading figures, similar to the various "passing 6/4-chord" idioms that also apply to this style? I intuit that this is so, but Kennan doesn't go into any further details.