The biggest problem I have with MIDI sync is that the latency is highly variable to the same device. It can be anything between 10ms and 40+ms. Whenever I start the sequencer in Reason, I have no idea how well the device will be in sync. I'm unable to see any pattern to it either.
I've been testing this specifically with an Elektron Machinedrum, which otherwise syncs beautifully to Logic and Live. I've also tested this with identical results using Monomachine, Analog Four and both MOTU and Focusrite interfaces.
I would like to see in Reason...
1. The ability to set a negative clock offset greater than the audio interface's expected latency. I always need to have the offset roughly double my stated latency to account for the MIDI clock data going out and the audio returning.
2. An option to have the sequencer stop sending MIDI clock when stopped and to only start again when restarted (this is how Logic and Live behave). I understand the reasoning behind keeping the clock running even when Reason's sequencer is stopped (to keep synced effects etc. running at the same speed after the sequencer has stopped) but I think this may be messing up the ability of synced sequencers and drum machines to accurately pick up the clock sync when Reason's sequencer is restarted. Allowing the user to choose which way they would like sync to work (free running or sequencer controlled) should satisfy everyone and hopefully fix the unstable sync problems I've been having.
3. A display of the MIDI sync status and the ability to switch it on and off in the transport bar.
4. As others have mentioned, it would also be great to be able to send Sync to more than one MIDI device/interface.