Thanks for the good questions!
Risks: in the almost two years since first release, I haven’t heard of anything disastrous. You might check the negative reviews: they seem to be related to confusion about how to configure things, rather than defects in the plugin. I do ask you to make a backup first.
Size increase: yes, keys take SSD / HDD space. It doesn’t change the number of items in the database, only the size of the data structures. I think it’s about 25%, but that’s (a) hard to measure and (b) different on different sites.
Uninstall: Please read this. If you want the keys reverted to WordPress standard, you have to do that before deactivating or uninstalling the plugin. At any rate, it doesn’t add entries to the database, only data structures.