Unlimited request business model for agencies. Pros and cons for founders?

Hello all !

Me and a freelance friend (both more than 10 years experience in design) are exploring some options in partnering up to catch bigger projects and exit the "individual freelance" trap.

As the field is ultra competitive, one option is to spice up the business model.
One interesting possibility, is on the unlimited requests model.

The client pay a flat fee, and during a duration (usually a month) he can have unlimited requests, treated one by one.
One successful example of it is https://www.superside.com/

Although, we do not want to take the same kind of work, the idea behind it seems appealing, and seem to gain traction.

As a freelance and an ex-employee of design agencies it makes sense because scoping of mission is always critical, and in my experience is more often than not wrong. (especially between sales and experts)

• Either the project was under scoped, and we end up loosing money, or need to go through a painful renegotiation.
• Or on the day rate / hourly rate it can be stressful for client as they go over budget if the team is not delivering as promised.

So I'll love to get feedbacks from founders on externalising work:

• Why an unlimited business model for a research x product x design agency would be interesting ?
• How would it change your life compared to how you externalise work today ?
• What fears would you have about such a model ?

Thanks !

ps: If some freelancers or agency owners are on the same mindset and read this post feel free to comment and add your thoughts on it too :)