MFU is good, but not always perfect for every booking.

CW availability is needed, so when that is non-existent you can't MFU. CW availability can be awful on some routes. CW availability can vary from being 'open' on a Tuesday to almost never on a Monday / Friday; especially on biz heavy routes.

MFU is less good once the cheapo WT+ tix are gone (or inside the low fare window). Its still doable, but maybe not so great as an I fare would have been a couple of months back.

In short, good when it works ... but not something to rely on to work to specific dates or on certain routes.