That depends on the breed and what your definition of rare is
I have lots of rare breeds I don't have any issues with most of them.
MFL very hard to keep chicks alive
Tolbunt Polish.... can't keep those chicks alive either.
I have
CCL... they do fine
Rhodebars They do fine too
Eng Orpingtons do fine
ALL of my Marans do fine (Isabelles, Cuckoos....that is all that might be RARE)
Choc and Mauve Orpingtons and Wyandottes do fine.
Can't think of anything else I have or had.
That sounds like me had trouble with those your whole thing there is about what I had then sold