Btrfs delayed allocation tends to combine small random writes into larger sequential writes anyway, so there may not be a significant boost in performance by using an SSD as cache, depending on the workload and depending on the cache settings.
If you expect a file system to survive crashes, no matter the failures of the SSD caching device, you need to use write through caching, i.e. writeback is off. Since writes must be stable on the backing device in write through mode, you lose the benefit of caching for writes, but you still get them for reads.
If you enable writeback caching, and the SSD doesn’t consistently honor flush/fua, and you happen to get an ill timed crash or power failure, the file system will be lost.
There is a non-subtle complexity with additional layers. Not just the software layers, but when combining SSD and HDD which have ambiguous and invisible firmware behavior.