IMO, flourochemical protectors work well on all fibres, but as polypropylene does not accept water well, any applied protector applied on site will not adhere to the fibre.
When applied during manufacture, protectors are cured with heat and this tends to adhere better to polyprop, so the carpet will then have an enhanced dry soil and oily soil performance. The polyprop is already highly resistant to water based soiling. The protectors we apply cure to the air.
Other synthetic fibres will benefit from protection to varying degrees, but I would still apply them to all of my own carpets regardless of whether natural or synthetic.
Safe and happy cleaning
The Ken