It is the best pc emulator ever.
Only one on the market that runs virtual hardware with proper speeds: disk IO, cpu speed, cdrom/floppy speed, cpu cache speed, ram speed, serial speed, cpu registers speed etc etc)
Accuracy was one of the major reasons for 86Box forking from PCem. PCem's development was spearheaded by a developer that cared more about playing specific games and took shortcuts to get there.
86Box is far more focused on being as accurate to real hardware as possible, allowing all kinds of old software to run, even the hard ones like OS/2 with its heavy uses of ring 1 and 2 security contexts that are usually entirely ignored by OSes like Windows and Linux (or, well, DOS, where literally everything is ring 0...). It can even run the 8088MPH demo made for the original PC :-)
With this shift in development focus, it pretty much necessarily sacrifices performance for the goal. They are often incompatible goals. See how emulators like Nesticle could run just a handful of NES games on a 486 compared to later developments like Nestopia that demanded a Pentium 4 to run full speed, but do run every NES game ever made.
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.)
For DOS, IMO, it's overkill. DOSBox-X does a very good job even for late and heavy dos games and is far more performant. A big advantage of DOSBox is not needing to setup a real OS inside the emulator - things just work.
For Windows things are the opposite, however. 86Box's better emulation of real hardware makes it far easier to setup the drivers and in general make the OS work well (on dosbox there are quite some quirks last time I checked, essentially requiring you to follow a specific guide, tweak some settings etc; on 86box it's just good old "install the os, put on the drivers and you're good to go"). Also, I notice that 86Box vms tend to be considerably faster than real hardware of the same level (likely will not be important for most games).
