Great question! Alot of the crowdfunding promotional companies and consultants on Kickstarter, Indiegogo, etc. are legitimate but it seems the majority of the ones that spam the users directly aren't. Either they are selling a good service for way to much money, or they are selling a useless service for cheap (i.e sharing your campaign to 100k fake followers or directing fake/bot traffic to your campaign).
TDLR; it's hard to distinguish the legitimate services from the scams.
If you have any crowdfunding services you are interested in purchasing, post here and I'm sure someone with experience in using them will let you know if it is worth it. I do also offer my own crowdfunding promotional services and you will be able to find a link to that in my signature below
Have a great day!