>"It is entirely possible that I have no idea what I'm doing with ads."
If you've managed the ads with a total time investment that equates to the equivalent of < 1yr full-time work, this is likely the case. Keep in mind that when I've trained dedicated digital ads people of all levels from entry-level to managers, I would never let someone with < 3-4yrs full-time experience run an account on their own.
Again, not a ding at you, but it may be helpful to replace "ads" with "software development" when thinking about how much you might expect out of someone's coding and architecture skills if they had the amount of experience equal to the time you've invested in learning and managing ads, and the projects you've managed them for.
For solo founders who have a substantial amount of hands on digital media management experience, I would absolutely recommend it to them if they felt their market research had shown a potential for them to be successful.
>"a confident marketer should at least consider something else than a flat rate. What exactly is the risk here? There is organic growth, there is customer retention, there are MRR numbers." .... "everything else is known and very clear." ... "what happens before a customer learns about the service, which in my opinion is entirely on the side of the marketing person"
The risk is it not being worth the return. Full/partial commission models for marketing are a question of who is shouldering what risk, and the balance against potential rewards. On the full commission side, depending on what resources you provide, how much control you give said marketer over your site, your in-product funnel, etc., they may demand 100% of first year revenue, up front, or more depending those variables. If they are fronting their own capital, they need to feel it will be worth their while for their target margins after paying media dollars, any creative/content/web production costs, taxes, etc. The premium that carries is often hard for people who are not marketers (particularly technical solo founders) to stomach. It's much the same as the reaction many technical folks have to "idea people" that want them to invest their time and energy into building the entire product for negligible equity. Just isn't worth it.
Partial commission, or performance-based bonuses can be an option. But again, it comes down to the risk balance. Are you supplying the media budget? Will you get them whatever data they need for analytics? Will you make changes to your product and funnel if they find traction with an audience that the product UX doesn't support well or feel your current marketing site sucks?
These days there is so much that happens after someone learns about a service that is mission critical to converting to sale. Unless you are willing to pay them solely to deliver on what happens before someone learns about the service (ex: a CPA for a free trial), you shouldn't expect them to shoulder the risk for what happens after someone learns about the service.
If you're interested in debating further, I'm happy to take this offline (contact in profile, I'm not trying to sell anything). Nothing wrong asking the questions you laid out, but I think solo technical founders are really going to struggle to attract quality marketers with this sort of mindset and assumptions.