There are no side effects in your build output.
For the both settings, it is very easy to put in settings that will take up 100 of your CPU.
Visual Studio by default turns on /GM but in reality, /GM does almost nothing.
The benefit of using all your cores vs. only compiling a of the lines in C++ file is no contest — on a multicore machine, /MP wins every single time. I haven’t used a single core build machine in a decade or more.
And as our esteemed forum member dot squid points out, there is less time for tea (or coffee, in my case.)