That is a good point. It looks like they have a $4999.99 limit in any seven day period [0]. That is also the single-transaction limit (though there are some variations if you are doing transactions with a merchant and not another private individual). Zelle has higher limits (over seven days, at least), but more of them. $3500 in a day, $10000 in a seven day period, or $20000 in a 30 day period [1].
So for certain high-dollar cash-equivalent transactions, these might not be the best choice. Though personally I think I'd probably end up using something more formal if I were transferring that kind of cash in a single private party transaction.