I forgot to mention liquidity which is something many people talk about.

I bet on both thoroughbred and harness racing in Australia. Many harness races have all the money on the first two horses. That's fine for me as I am only betting on the first two faves.
Sometimes I come across a race from a smaller venue that has only a few hundred dollars bet on it with a minute to go. I leave those races alone.
If there is $10,000 on the race at the time I am looking, that usually is enough to let me have a bet if I stick to the fave.