It is obvious that with a new approach (Photos) one was conjured up new compromise situations again.
I think Apple really wanted to implement a new open, efficient and unlimited library structure, and in many aspects they succeeded. The speed at which huge libraries are processed is simply breathtakingly good.
And I have to agree: the slideshow issue and themes are really still amateurishly solved. Also all other available slideshow programs are really silly and completely inflexible. I love FCPX, at this GUI level Apple could also develop a fantastic editable workflow for slideshows. Are there no good software designer or team leaders in California anymore?
The existing templates (e.g. Magazine, Sliding Panels) are really high level, but iPhoto had more great ideas implemented!
Especially for travel documentation I remember geotag/GPS sorted travel presentation, something really revolutionary that has never been seen since.
Apple's add-on architecture hasn't been taken up by external developers either, so it's all very sad in this sector. I assume that the iPhoto development team had left Apple because some braggart new development manager for Photo scared off these good people. A sad chapter. If there's anyone who knows more to know, I'm interested.