I do a lot of volunteer work in Guaymas, Sonora. Often times I am the only American/Caucasian/English speaker. I have never been treated poorly. I understand and speak Spanish more slowly (obviously) than the people I am working with so sometimes I feel like I am on the fringes of the activity or just working alone in a group. However, I have always been treated with kindness.
I did have an experience in a local pet store once. I asked the owner a question (in Spanish), and his response was too fast for me to understand the first time. I asked him to repeat it. He immediately switched to English and berated me, lumping me in with “all Americans” who tell Mexicans to speak English. He suggested I learn to speak better Spanish. I felt very defensive and continued speaking to him in Spanish, explaining that I did have a teacher, and was still learning. He wasn’t interested in my excuses. I was embarrassed to be singled out like that. I was ashamed that he probably had a very bad experience with another native English speaker that caused him to react to me this way. At the same time, I understood exactly why he did what he did. I understood his anger and frustration. In the end, it was a very good experience for me to have. It helped me be more aware of how others may feel in similar types of situations. I am more empathetic as a result.