I can't really speak for someone with a library that large, although I migrated my library from iPhoto (having tried Lightroom for a bit) onto Google Photos. I've been very impressed.
iPhoto was good to an extent, although it became cumbersome to manage as my library grew, and the file structure kind of became a mess after migrating my library several times. I love that Google Photos has unlimited storage space and I can access it from any device with an internet connection without having to take up much physical storage space (except for some cached files). It also does a good job at organizing all of my shots automatically without requiring any time from me. It makes cool little collages and videos as well, and I never have to worry about clutter since it deletes duplicate images and has unlimited storage.
I would say though that it works best with personal photo libraries. If you're a serious photographer who does photography as a hobby or for a living and generally have lots of photos that are for clients and not necessarily your own personal photos, you would probably not want to mix up those photos together. I believe you can make albums and such in Google Photos, but I generally let it do all of the work for me.
You are limited to 16 megapixels per photo and 1080p video under their unlimited account, although that's perfect for me since my camera's resolution is under that.
