There are some other downsides, such as W3W being proprietary and based on (IMO) bad design choices (e.g., hard to localize).
For more on this, and why I think we shouldn’t advocate for W3W, see: https://shkspr.mobi/blog/2019/03/why-bother-with-what-three-words/ for theoretical reasons and https://w3w.me.ss/ for some practical examples.
As you mention, https://plus.codes is indeed much better, although this is only tangentially related to your project
For more on this, and why I think we shouldn’t advocate for W3W, see: https://shkspr.mobi/blog/2019/03/why-bother-with-what-three-words/ for theoretical reasons and https://w3w.me.ss/ for some practical examples.
As you mention, https://plus.codes is indeed much better, although this is only tangentially related to your project