Feature-wise it's identical to this other sitemap extension, the main difference being that this one doesn't write anything to the filesystem. That's an advantage because this way you don't have to deal with annoying permissions issues on the filesystem.
I tried to go with an existing sitemap library, but as it turns out they are either very outdated and not published to Packagist or very recent but too tightly integrated with Laravel. In the end I re-implemented the logic by copying small bits from those libraries, which wasn't that hard to do.
I first tried to cache the sitemap output to replicate the file-on-disk behavior, but after some thoughts I decided it wasn't worth it. The queries to generate the sitemap aren't that complicated. More complicated queries are run everytime you load the homepage. So I'm just returning a fresh and up to date sitemap every time you ask for it.
If there's interest I can implement the links limit (number and file size) according to the sitemap specs. The specs says there shouldn't be more than 50k links per sitemap file. For now it will keep growing forever. I'm not sure what is the actual search engine behavior if you exceed 50k links per file.
Other features that could be added are the ability to select what type of page is added to the sitemap, an option to choose the sitemap index file name/url and an option to cache some of the output for a period of time.
Don't hesitate to open an issue on the repo with your wanted feature and use case if you want to discuss a suggestion !