Short version is that ZFS is using ubiquitous battle tested industry standards in symmetric crypto and there is nothing to worry about there, barring someone discovering a significant implementation error. An external security audit would be nice down the road, but if we're comparing to VeraCrypt then theirs a year or so ago didn't inspire confidence (though it did prompt some improvements).
The specifics of the OpenZFS implementation comes from the Illumos project (specifically the Illumos Crypto Port), and takes into account some specific needs for ZFS such as the ability to do scrubs or raw sends without keys. If you want a bit more discussion at length, you could read some of the OpenZFS native crypto issue discussions on GitHub, such as #6707,
ZFS native encryption, GCM file size limitations, questions on best cipher encryption mode.
A few notes on your comment otherwise:
whatever wrote:Truecrypt has cascaded encryption and RIPEMD-160, SHA-512, and Whirlpool
First, none of those are crypto algorithms (ciphers), they're all hash functions. The ciphers in Truecrypt (now VeraCrypt) are ones like AES, Serpent, Twofish, etc. AES is a completely standardized rock solid SP-network cipher dating from 1998 that is widely deployed. While not the fastest or simplest in principle in practice hardware acceleration instructions under x86 (in the form of AES-NI) are also effectively ubiquitous on any remotely modern hardware. Intel first introduced them in the Westmere series Xeons (56xx) IIRC, back in 2010 (these are the processors you'd find in a 2010 Mac Pro). It was added to desktop and mobile chips with Sandy Bridge. From a Mac or even Hackintosh (and thus O3X) perspective you're near certain to have it.
AES-256 has no known security breaks, and while it is trickier to get right (when not using AES-NI in particular) vs side channel attacks, those are generally irrelevant in the specific application of FDE whose threat model is solely against cold physical attacks.
but I'm not familiar how that would stack up against the ZFS aes-256-ccm / aes-256-gcm. Can anyone offer insight ?
VeraCrypt uses AES (with hardware acceleration when available) too, as well as others. However cascading ciphers is of dubious value, or even negative value in that it adds unnecessary complexity and possibilities for foot-gun. In the VeraCrypt audit for example an issue came from the use of the GOST 28147-89 cipher which cannot effectively be used with XTS, that's both a silly mistake but also one that came about from having GOST (which is fundamentally flawed) at all. IIRC they dumped it afterwards, but it illustrates that more is not better, it's both slower and adds code complexity for no reason vs a good cipher. The OpenZFS implementation is designed to be easy to add new checksums/ciphers too should that ever prove necessary.
So it's unlikely there are any particular issues with OpenZFS vs anything else when it comes to security. There is the caveat that it's relatively new, but the primitives are not and it's coming from the work in Illumos.