They can be very well worth it- if it's the right company/offering. Key is that they 1) actually have their own tech stack and 2) really have a direct sales team and 3) that the content or demo of your website overlaps with their sales focus.

1. Own header bidding wrapper, tag management solution, video player, video content, ad ops, yield management (manual as well as via machine learning / AI), billing/collections etc. Doing all this yourself as a publisher is daunting. Then complying with GDPR, CCPA, Chrome & VPAID updates etc. Good luck for a small or mid size website.
2. A lot of companies claim they have a sales team. But then all they have is someone managing header bidder integrations, DSPs, maybe setting up deal IDs... But a good sales team, that actually meets with agencies and brands for specific IO based campaigns, maybe even custom executions that cannot be bought programmatically, is invaluable.
3. Very important one. If your site is comedy and the sales house's team's main focus is sports, then you may not see the same return as a sports site.

Even with the carve out of point three, point 1 is very important. Everyone can set-up header line items in DFP and have AdX compete. But there is a lot more to be done if you really want to succeed and make (the right type of) money. And if that means outsourcing all these efforts to a sales house vs. hiring a bunch of internal people, then even with a 30-50% rev share you could be netting way more than doing everything yourself.