86Box forked from PCem specifically over issues of emulation accuracy. The developers of each had very different motives. PCem was aimed at running a few select games as quickly as possible, while 86Box is aimed at accurately representing real hardware behavior. (I do use past tense purposefully: PCem for all intents and purposes has been abandoned.)

DOSBox is meant to be a lightweight DOS runtime on top of a host operating system with the minimal hardware emulation necessary to accomplish that. Generally it makes running games easier as you don't need to deal with autoexec.bat, config.sys, and all the memory management that comes with real DOS. Under 86Box, your VM is yours to meld in exactly the same way as a real old PC; if you run MS-DOS, that means all the nasty parts come back. (I personally recommend installing Windows 95 at least, running DOS games under 95 tends to be a big relief for these same reasons.)