I don't think this is true of "proposals" in the plural. This is arguably true of the California ballot measure due, allegedly, to sloppy drafting.
But this isn't a fundamental aspect of wealth taxes; it's a choice. So I don't think it's a meaningful argument against wealth taxes in general.
(As an unrelated point, I think dual class shares are sort of bad and it makes sense to me to discourage them as a matter of public policy, but this doesn't seem like necessarily the best way to do it.)
It seems to me that the authors were, as you say, trying to close a loophole, namely that where I set up a Nevada LLC to which I hold 99% of voting shares but 1% of outstanding equity, put my assets in it, and just direct the LLC to spend on things I want it to--but in the process, they failed to realize that non-public super-voting shares (like Google class B shares) seem to fall into category (3) of section (c) (i.e. neither sole proprietorships nor publicly traded assets).
I don't think that's a great design, though it's still worth noting that it doesn't rule out privileged shares in general--GOOGL, to give an example, are privileged voting shares that trade publicly, and thus are obviously under category (1), and assessed per their FMV.
But in any case, I stand by my broader point: this is a specific detail of how the California proposal has been written; it's not a fundamental aspect of wealth taxes. Switzerland has a wealth tax that does not have this aspect, for example.
But this isn't a fundamental aspect of wealth taxes; it's a choice. So I don't think it's a meaningful argument against wealth taxes in general.
(As an unrelated point, I think dual class shares are sort of bad and it makes sense to me to discourage them as a matter of public policy, but this doesn't seem like necessarily the best way to do it.)