Conservative collection is unfortunately terrible when you're dealing with data that looks random, like cryptography. It generates many false "pointer live" positives, which ends up retaining a lot more memory which looks like it's leaking.
True if you don't cooperate at all with the collector, but easily avoided in many cases. E.g., when using Boehm, try using gc_malloc_atomic() for data that you know cannot contain pointers, like your encryption buffers.
