A tack bra is very painful all the time from the minute you put it on to the minute you take it off, and in fact even for a while afterwards. And while it's on you'll feel it with every. Single. Little. Movement
Very effective but difficult to not show the discomfort if you're wearing it all day.
A crotchrope under your clothes will be just as noticeable to you but slightly easier to keep secret... That said, I have managed to survive a full day at the office in a tack bra that included a client meeting and a grocery shop and meeting my husband for dinner afterwards, so it can be done... Just depends on your threshold I guess