The 1.27mm thickness is Apple's specification. Thicker SIMM PCBs can bend
the retaining springs out. This isn't a problem as long as thinner SIMMs
are never inserted into the same sockets.
If thinner than 1.27mm SIMMs are used, the chance increases of having weak
or impaired connections.
The main point to remember is that Apple's sockets are for 1.27mm SIMMs. If
you buy SIMMs from Apple, you can be sure they're OK. If you buy from a
third party, be sure to ask for SIMMs with the right thickness.