When live musicians play they keep an ear out for the congregation - did they come in at the right time? Are they keeping up? They can respond appropriately. However, a backing track cannot do this! It continues on regardless. A worship leader and/or singers (preferably with microphones), become more important than ever.
Singers, know your Trax
Those leading need to know the backings well - really, really well. They need to know: exactly when to come in, the structure, if there are bars between verses, and to know how songs are going to end. It is difficult to fall behind the beat of an accompaniment, unless it is far too quiet: lean into it and it will support you.
The backings need to be heard, don’t be timid with the volume - a common mistake when first using them. If they are not loud enough to keep everyone tightly in time together, don’t bother. If your MIDI gear enables you to remix parts, make sure drums can be heard as they help keep the time, and bass as this will give depth to the sound, but go easy on parts which are playing at similar pitches to the singing - leave room in the mix for the congregation!