The ECU will usually work, although maybe not too well, without the caps. Failures usually occur when the tracks become damaged or the cap goes short circuit. It is possible that although the cap was leaking it may not have been the cause of the failure. The 12V supply to the injector is switched by one of the transistors (2SB1020) on the heatsink, that transistor could be faulty as could the drive to it.
The ECU is susceptible to damage from static discharge, you should always wear a wrist strap or you can destroy it just by handling the PCB. Usually though ESD damage leads to premature failure rather than instant failure.
