There's been much praise for APFS published here and elsewhere, but since I haven't seen anything that addresses potential down the road problems with it I'm wondering if the new file system presents vast, new opportunities for the bad guys, or is it only the OS, itself, that's vulnerable to exploits?

Interesting questions. I suspect it's only the OS that is vulnerable, but we won't know until it is unleashed on the world and all those nefarious characters start playing with it.

I have said before, "any protection the human mind can conceive of, another human mind can find a way around" so there is no absolute security. However, the "bad guys" would have to get past MacOS to get to the APFS file system and certainly APFS is far more secure than HFS+ because of how files are physically stored on the media and strong data encryption is designed and built into APFS — not an add on as in HFS+ — would appear to make it a significantly harder target.

That's the answer I was expecting, and it's a good one.

And it suggests to me that it might be wise to wait a while before jumping on the APFS bandwagon, either by avoiding High Sierra altogether or by running it on a SSD formatted HFS+ (which I believe is possible).

If the developer beta 8 installer is any indication while you may be able to RUN High Sierra on an internal SSD formatted HFS+ you will not be able to INSTALL High Sierra directly onto an internal HFS+ formatted SSD. (No HFS+ option was offered by that installer on my new MacBook Pro.) However, you should be able to install it on an external HD and clone that to the internal SSD formatted HFS+. You might have to forego the benefit of a Recovery Drive on the internal drive in that case. I am still unclear on how CCC handles that.

We've reached the exact same conclusions about installing and running High Sierra.