They can send the actual voucher to your email; I have one from yesterday that came via email, text, and app. The image in the email had the card #, CV2, expiration date, and billing zip--no need to go beyond the email to use it.

However, I have experienced exactly what you describe, too... had several flights at the end of 2023 which were delayed and in some cases looked like they were going to misconnect and strand me, and each time, I got the link to get the food voucher... and clicking it either indicated it had already been claimed (?!) or else took me to the flight disruptions section of the app (and the web page, when I tried using my laptop) where there was no option for the meal voucher, and the rather unhelpful "please call UA" message rather than links to search for alternate flights that are usually there. Worst was an ORD-IND flight where the aircraft was stuck in ICT due to severe WX and was going to be delayed ~12 hours; none of the automated stuff worked other than to tell me I had a flight disruption and to explore options (none of which were offered). That one had an "already claimed" error when I tried to access the meal voucher... fortunately worked out alternate flights during the IAD-ORD flight via chat in the app.