Can ACH not be reversed? My understanding is that the European systems are just as vulnerable, but what makes them "secure" is that they can be reversed no-questions-asked, making such an attack pointless unless you know the account holder isn't going to notice it for months.

Not sure what you mean by "reversed". You have 60 days to tell your bank the transaction wasn't authorized, iirc, and you should get reimbursed. It might just be a hassle and it likely wouldn't be quick.

I've only had to deal with credit card fraud in the US and it was easy enough.

I did have a restaurant accidentally charge me $983 instead of $98.30 on a debit card for a meal during a holiday and, even though they immediately voided it, that still ended up basically blocking almost $1000 for several days. I can't imagine reversing an ACH transaction would be faster.

Overall I have no huge complaints about banking in the US. I just find it better in Europe so far, particularly sending money with IBANs.

No, European systems are not vulnerable like this. You can't do anything given my IBAN etc., you would need access to my banking app or website plus whatever 2fa I have set up there to send money from my account. And SEPA transfers can't be reversed easily AFAIK.