What do you think about robo-advisors charging a fee based on performance of portfolio?

I feel that its a better way to charge fees as compared to a fixed commission fees like how other robo-advisors are charging though.

Simple calcuation

Autowealth: 8% on __profits

Stashaway: Fixed 0.8% annually

If your portfolio has 10% annual gains

Autowealth: 8% of 10% = 0.8%

Stashaway: Fixed 0.8% annually

Hence if Autowealth is able to grow your portfolio by any % amount less than 10%, it is more worth it to park your funds with Autowealth as compared to Stashaway as the fees are lower than that of Stashaway's fixed fees.

And I don't think its an Apple to Apple comparison if you were to compare investing into ETFs v.s. Robo-advisors as one is non-actively managed whereas robo-advisors would actively manage your portfolios. If you were to rebalance your portfolio by yourself, it would be a lot more costly with the transaction fees involved