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