Run this code in singleplayer or just in editor's console:

doStuff = { 
diag_log str _a; 
}; 
 
_a = 123; 
remoteExecCall ["doStuff"];

"123" will be printed in the .rpt file.
If you run the same in a multiplayer session, _a will be undefined in the doStuff function, as expected.
If you replace remoteExecCall with remoteExec, it also works as expected (variable _a is not defined).

It can lead to unexpected behavior of functions that are meant to work both in SP and MP, adding complexity to multiplayer scenario debugging.
Obviously one would expect a remoteExecCall'ed function to be always run in a 'clear' environment.