Wow, I must have hurt some fee-fees.
Acorns targets young investors without a lot of money to invest. I believe the way it does so encourages the wrong thinking about saving and investing. I'd rather see a young person save up the $1000 or $3000 account minimum (aka "real money") and open an account with a traditional investment company like Schwab, Fidelity, Vanguard, or one of the other excellent brokerages out there. I think it's preferable to consciously save towards a goal than passively letting a service do it for you. The behavior underlying saving is the important thing for young people to learn, and Acorns specifically doesn't really help with that by putting it on auto-pilot.
If rounding up debit purchases materially affects a persons savings rate or investment returns in any way, then there are probably some behaviors regarding saving and investment that need to be improved. Relying on gimmicks like this just seems like such a warped way to handle money that I honestly wonder who it would have any real benefit for.
Also, if you're not a young person and you have enough to open a brokerage account, why are you paying Acorns fees? 25 bips on $5k or more is a lot to pay for what amounts to ACH transfers.
It's probably immaterial. I doubt Acorns is profitable and I doubt they'll be around in five years.