Your right Sportz,
you have to have a cut off point somewhere and that's where mechanical systems fall down somewhat. When it comes to days since last start or barrier draw and I'm doing the form on a race [my preferred mode of betting as most here know], I take into account what suits an individual runner. Is it a swooper [outside half of the barriers please]?, an on-pacer or leader [inside half please]? and that too will depend on the track, race distance, field size and track cond't. Mechanical systems can't do that.

The days since last start rule depends on the horse. I do a history check and see what the trainer has decided suits the horse. If I see regular quick turn-a-rounds and then see it is suddenly 21 days since it's last run, my 'didn't pull up well or there has been an injury or some other issue', alarm bell rings. Usually I have saved my money by not betting the horse. Some horses run only once a month because that suits them.

I think any system can be improved immensely by doing a little investigating on the selections it throws up [that's why I advocate handicapping as the best selection system of all]. The last rule of any system should be: 'Handicap the selections'. It doesn't defeat the purpose, because the purpose is money in our pockets.
Yesterday was a classic example of how that works. 5 selections for one of my systems. A quick check showed one selection with an excellent win rate at 1000m and 1100m was trying for the 1200m for only the second time [hopeless first time]. The runner was 5yrs old. Only the 2nd. time at 1200m?
I didn't back it and it was unplaced. Another runner was coming back to 1200m from 1350m. No history of winning coming back in distance so no bet and unplaced. 3 only bets for 1 win at $5.40 and 2 seconds from 3 bets rather than 1 winner from 5.

There are no rules that can be added to any good system to let you know when a mechanical selection is a stupid bet. A quick handicapping of the selections sure can. Sometimes I get it wrong but not often, as the the reason for ruling out any system selection is generally easy to spot, not deep and meaningful. "!0 starts at this track and never placed" ? Cross that selection off as a bet. Obvious things. So why do systemites insist on Lemming like bets just because the system tells them too? Dumb and dumber or haven't got a clue about handicapping?
When I bring up the idea of [quick] handicapping system selections as a great way to improve SR, I am usually deafened by the silence, so I suspect the latter of the above reasons mostly applies. It's an observation, not a criticism.

Cheers.