Over the past 15 years and well over half million miles of vacuum door lock usage, I have seen the systems to be ridiculously reliable. The few times that I have had a problem, it was extremely easy to troubleshoot, replace the part, and the part was very cheap both times.
Also, to use electrics on all doors plus the trunk and fuel filler that lock automatically when the driver locks his door would be a problem.
As a matter of fact, I was disappointed to see them go away from the vacuum system on the 203 car, the electric locks are noisy, the vacuum locks were absolutely silent.
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in