Well, it is common mean that C is universal language; but this is a myth at least for the point mentioned by you placed over MCUs.
Indeed, skeleton of algorithms and (sometimes) structuring at paper seems good -- but as long you do not come at programming routines. You may say: "what is a problem -- I just replace defines and headers and it does work on other core". Well, even 8051 cores have different ways to do the same.
For example, you said "CAN". I shall not talk about that different tranceivers have own signaling/timings (this is different from chip-to-chip so they are hardware ones) but general usage is different due cores` realizations. I see no different between you need to re-write your code in C due [total nighmare] unspecified (expansion, core-dependent) libraries in C and new subroutines in ASM in terms of matter.
What I wish to say: you may easy transfer a C code from x86 Intel^(R) to AMD^(R) but C51 to ARM takes a lot of thinking due real hardware realization. I do not talk about printf(), atof() etc. But even for "the same" cores think about, for example, how do you process BOFF mode flag due conversion from Atmel CC03 to Infineon (oh, there is no standard library/function for this in both seem same cores). Anyway you need to do some hand-work -- there are many not-ANSI waypoints. Sometimes it is better to use even read-to-use ASM routines to do the same when conversion than try to imagine what is __ARM_blablabla() means and which __C51_i_try_do_same() does seem same do.

Regards,
Oleg