The capability to switch to FBReader is unique to PocketBook, but it has made them lazy about improving their implementation of Adobe Digital Editions. The best implementation of mobile ADE for ePubs is on the Bookeen devices, which do allow control of fonts and other styles via menus. Note that, even if they don't want to add bells and whistles to Adobe ePub support, there is nothing stopping PocketBook from using a better font than the one provided by Adobe as its default ePub font. In addition many other implementations of mobile ADE allow other fonts on the device to be used without embedding them in the ePub. In most cases this requires updating the CSS in each individual ePub (not possible with DRMed ePubs), but there is a hack for the Sony Readers that allows for a generic user-provided CSS override that works for all ePubs.
I suggested PocketBook improve their ePub capability a couple of months ago, but I don't think they even understood what I was talking about, see
Newcomer. They obviously don't have to take my suggestions, and overall they have good firmware, but Bookeen is already way ahead on Adobe ePub support and Onyx looks like a serious competitor for the "best with FBReader" crown.