EPub is supposed to be the general purpose generic format for ebooks, so if a particular machine won't play it, that'd be a big black mark against that particular machine more than anything.
There's a lot of shinannigans going on these days with using proprietary formats to prevent other machines or apps from using a given companies ebooks (in attempt to force platform dependency on the users). Generally, it's to company A's advantage to make their own devices/apps able to use as many formats as possible, while simultaneously preventing company B's devices/apps from being able to use company A's native formats. Basically it's a big anti-competitive monkey knife fight over who gets to have their vigorous and sticky way with the end users. Since Epub is not attached to a specific company or device/app, there's no one to block other companies from using it and thus it should be compatible with nearly everything, e-reader wise.
Some of these formats, like Kindle format for example, are really just ePub with a DRM wrapper.