My only issue with mechanical systems is that they tend to focus on what a horse has done in its previous starts rather than comparing its performances relative to other horses in TODAY's race.
As an example - if today's race was a 1350m race at Doomben and your system rules included (say) "winner at track and distance". The system would select a local horse. Unfortunately that sneaky Mr Hawkes had shipped up Lonhro for the race - never raced at 1350 or at Doomben so would slip through the filters. System falls over.
I guess that's where judgement rather than blind acceptance of the system output comes in.