At the time of its introduction, MIDI was quite OK. However, modern synthesizers and drum computers are 64 or even 128 voices multi-timbral, so they invite for a lot of data to be sent to them. At a datarate of roughly 32 kbits/second on the MIDI bus, it is quite easy to hear that delays are introduced when a lot of voices are overlayed. For professional applications this was unacceptable. Some 10 years ago, MIDI-2 was introduced, supporting a higher bitrate (64 kb I believe). It never really made it in the home synth market.
Most modern MIDI programs nowadays allow several MIDI ports to be controlled so that the data is spread over several buses. MIDI analysers (built into the software) can detect possible problems due to data pile-ups, but making a proper instrument scheme is still something the 'composer' needs to take care of.