I'm sure that C is a better solution, 'cause, it's more portable and more adaptable... One ask me if
SLIP can be used instead PPP, with my layer hierarchy, only one module and some CALL must be changed...
All my code work around µC specific library i write (like string and memory block manipulation).
These can be optimized in assembler (or use compiler implementation (strlen, memcpy, ...)

for me, it will be an error to write all state machine in LCP/IPCP or TCP in assembler ... but it's my mind ...