How can this circuit become more robust?
1) LED lifetime is related to thermal, the higher the temperature the shorter the life. Make your LED's happy by giving them less current and/or cool them off.

2) If your worried about voltage spikes, then find a zener diode and clamp the voltage across C4. Make sure the zener is going to be able to handle the spike and not blow up.

3) If your LED's have significant voltage drop differences (some have a 3.3V drop, and some have a 3.6V drop) some of them could be dissipating much more power than others. If this is happening, you will have to match them.