According to the study, most of the participants earned high education and owned smartphones. However, the study did not mention about the GPS watch ownership in the participants. They might not be familiar with the GPS watch and had difficulty to use it.
Both smartphones and GPS watches have advantages and disadvantages. For smartphones, participants were familiar with them and they were easy to use. But they might not take the smartphone to every place especially in their household. For example, they left their smartphone on a table and went outside to do the gardening. For GPS watches, they could wear it on their wrist, so it could be carried to most places they went. However, the font size of letter in the GPS watches might be too small that elder participants had difficulty to read.
In conclusion, the information in this article may not be enough to consider between using smartphones and GPS watches.
To answer your first question, there are other factors affecting consideration to use location tracking research in the future. One of them is data security which is one of the factors that participants need to know and to be ensured that their information is going to be used only in the research.