I fly IAE equipped 320's..
We have auto-start that requires monitoring only starter time limits should an ECAM require cranking.
If Auto-start is available, it really does take about 5 seconds to initiate the start; and the after start flow takes about 10 seconds... Very easy.
The manuals we have require a 3 min warm up if the engine has operated in the last 2 hours otherwise it is 5 minutes; however our S/E taxi out procedure calls for start initiation 10 mins prior to takeoff as an additional layer.
Airmanship points.. If for any reason the workload is higher than taxiing in a straight line with an automatic start, you shouldn't be taxiing.. so stop! Tell ATC you need 3 mins at the holding point before ready. If there are expected difficulties on the taxi or you have relevant defects, keep a standard two engine taxi.
The threats with SE taxi IMO are forgetting to check the ground crew are clear, forgetting engine anti-ice, tight left turns and anyone in the other seat with low experience (to monitor me or the start).
From a commercial perspective, if you're flying older machines you might not detect faults until at the runway which at most major ports will mean you can't get a gate on your return (outer bay only) where as before you taxi you can usually reclaim your departure gate.