What are your reservations about a B-corp?

Nonprofit legal status is hardly a guarantee of anything about corporate behaviour, aside from what paperwork they fill out.

To be clear, I'm actually curious about it, especially because Bs are becoming mildly trendy, and I don't know that much about them.

I've got nothing against b-corps. In fact, I think they should be the default corporate form rather than one you need to opt into with specific language in the articles of incorporation (as in Delaware). But b-corps are still owned by investors. They are legally entitled to take into account considerations other than maximizing shareholder return. Whether they do or not is a separate matter; they are not obligated (although this can depend on state and type of entity, I believe). They receive funding as investments (with expectation of returns) rather than donations. Again, no problem with that as such. But if my choice is between government operated petition site and a petition site accountable to private investors (even if a b-corp), I wouldn't reflexively favor the latter.