I have been batting around .500 in being offered vouchers in the app. I had a delay on Friday that claimed I'd receive vouchers, but all that it offered was rebooking options. (Which is better than a prior similar delay, where the rebooking link didn't work and sent me to "call reservations" every time.)

I've received them for flights that made zero sense, including flights I'd already changed, then didn't get them for 3+ hour delays I sat through. I've given up on thinking there's any logic behind it.