100 % emulation is possible technically, but it'll not be viable on the performance side. The CPU needed to run it full speed (I speak for the Saturn, but I think it'll be the same thing for other hardware) isn't produced yet, and will not be until some years.
To quote Mame source (regarding the ST-V driver): "40Ghz CPU needed when the driver is complete".
So to gain performances, tricks are used (as detecting interruptions every XX instructions, instead doing it between each for example)
The accuracy isn't as good as on the real hardware, but it's barely noticable from the user point of view.
Some more techniques (as HLE) allow to gain a lot of performance, but the display accuracy suffers from the approximations (I'm thinking of UltraHLE)