There are fewer ways to game revenue share via accounting tricks or other shenanigans, whereas there's plenty of ways for insiders to extract cash from the business without it being classified as an accounting profit.
For example, suppose surplus cash flows generated from sales are funneled into large executive salaries and classified as operating expenses. The increased expense of executive salaries reduces the business' profit, which would lower profit-sharing liabilities but wouldn't change anything from a revenue sharing perspective.
Other dodgier ways could be related-party deals where the business enters into an agreement to buy something from an entity controlled by one of the executives -- leasing the company office owned by an executive for above market rates or so on. Cash is extracted from the business before being classified as a profit. But again, from a revenue share perspective, this doesn't change the situation.
It's way too easy to minimize profit (they're going to want to do that anyway to minimize taxes)... hire someone, raises, buy things, lease more office space, etc.
Take the money off the top (revenue) so that you're an expense from the start that they can account for and can't easily manipulate to avoid paying you.
In other words translate the phrase "profit share" to "voluntary donation" and the meaning will be the same.
If you want uncapped upside then it has to be based on revenue, but even then it can easily be manipulated (A sell to B for pennies, which you get a share of, B sells to the market for the real money.)
This is one of those cases where you should have a minimum-number in place, and you should be OK doing the work for just the minimum number. Treat anything else as Voluntary Bonus.
Even if the client is completely legit, the incentive is there. It becomes like spending pre-tax money, so why not incur some more expenses.
Businesses make 100 decisions a week on spending money. All of it comes out of "profit". Most of it is necessary. Some of it isn't. Some is ultimately wasteful but 'seemed like a good idea at the time.' Some is there to reduce profit so to pay less tax. (In good years it's legitimately good to replace ageing equipment, do repairs, pay staff bonuses and so on.)
In essence, by taking a profit share you are committing to be paid -last- the lowest priority of all. It's a terrible idea.
If you want variable returns it has to be based on -revenue- and even then you're relying on the customers goodwill.