Plugins based emus are generally junk for the reasons VmprHntrD said. The core programmers did a bit of work, but not enough to be solid, hence the plugins have to deal with that to do the best they can.
The problem with a plugin based emus comes down to the fact that the way plugins interact with the core program may change over time as that program evolves (and gets better/more accurate) which would make the plugin authors have to catchup. But keeping backwards compatibility for plugins may just lead to the same problems with emulation.
Most the really great emu's out there don't use plugins or only minimally use plugins where the emulation core can do most of the work on its own.
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