Let’s be real: yes, an outdoor bench with armrests is usually a problem if you want to lie down. The armrests break up the flat surface, so instead of one long space you get a shorter, divided area. If you try to stretch out, one armrest tends to hit around your hips, ribs, or knees, and you end up curling sideways or hanging part of your body off the edge. It is not comfortable for more than a few minutes.
The real trouble depends on the bench design. A bench with armrests only at the ends is annoying but sometimes manageable if you are short or okay with a fetal position. However, many public park bench armrests are placed along the seat specifically to discourage lying down. In that case, it is basically impossible for an adult to lie flat. Even a single center armrest can press into your back or force your legs into an awkward bend.
There are a few exceptions. Some wide benches have armrests set far apart, so a smaller person may fit between them. You can also lie on your side with knees bent, but after a while the hard material and the armrest pressure make it feel worse than just sitting up. If you carry a thick pad or sleeping mat, you might soften the surface, but you still cannot remove the armrest barrier.
If your goal is an outdoor nap or resting flat, look for a long bench without armrests, a flat concrete or wooden bench, a picnic table bench, or a park lounger. Otherwise, the ground with a mat is often a better option than fighting an armrest bench.
So yes, armrests are a problem for lying down. They are designed for sitting posture and often work against anyone trying to sleep or stretch out. Choose a bench without armrests if lying down matters to you.