As someone with dietary restrictions, it can be difficult to find food in F. The only time you are supposedly guaranteed a vegetarian option is on longer mid-con flights, where they mention on the United site that you have a choice of 3 options. Having flown SEA-IAH several times, I would say that's true about 25% of the time. Usually it's just 2 options, and often neither is vegetarian.

As mentioned above, bring your own food, or eat beforehand.

Not to take the thread off-topic, but having flown AA in F quite a bit this year, out of 40ish segments, I've only been on 2 flights where there wasn't a vegetarian option. Typically their 2 choices consist of Veg and Non-Veg, which I have noted is a stark contrast to United's choices.