I had some problems 3/17-3/21/2015. Mornings were fine at MK, Epcot, and DHS. However, afternoons at MK and AK were bad. AK was so bad that I had to switch to data, which was better. This is on an iPhone with ATT. I was using the app to try to change an FP+ attraction, while watching Flights of Wonder and maybe even in the EE gift shop. I actually lost trying to get a Safari FP. I don't know if it was the poor wifi or Disney's MDX servers were slammed. It sounds like they did an upgrade right after we left. This trip was worse than our trip in late July. I don't think I ever had to switch to data then.
EDIT: Don't rely upon a connection to know when your FP+ and ADR are. Either put them in your calendar, a screen shot, a note, or on paper.