well this is my reasoning - bear in mind my system (i've got 2) revolves around fitness.

Hurdles/Steeples: Racing is too irregular to study fitness

Maidens: No previous winning history to show when they're at their peak

2YO Handicap: Minimal or no form to calculate judgement

3YO Fillies: Races generally spread apart trained differently to Colts and Geldings. Fitness becaomes partly guesswork

Mares: Mostly race in open class races and the older they get the more reliable they become.

WFA: The best horses win them. Races are so spread apart that you can't get fitness again

Classes: How can you consistently make a profit backing poor quality horses


GOOD RACES:

Fixed Weight Open 2YO
Fixed Weight Open 3YO up to 1600m
Class 6 or better only 1600m
Welters
Open Handicaps through to Group 1
Quality Handicaps