Hardened Valve Seats or Not?

There is one risk not mention thus far concerning the installation of hardened valve seats.
I had hardened seats installed on my 65 273 and 2K miles later I had a cracked cylinder head at a seat.
I ended up replacing both heads without hardened seats and now just use lead additive.
Maybe bad machine shop work caused the crack, but seat installation can create stress that may lead to a cracked head.