My workflow is a little different. I can work in either Photos or iPhotos because my data resides on my desktop PC. When I'm working on a project like this, I select the photos I want to use on my PC, tweak them in Photoshop, then copy them to a flash drive and import them into Photos or iPhoto. I disliked the database setup in iPhotos, and long ago made the decision that I'd stick with my PC and the excellent file management program (Directory Opus) I use.
So, I don't need to migrate between Apple programs. When I'm done with a Photo or iPhoto Project I can delete it and start from scratch on the next project. I haven't deleted any project yet because I have had occasions where I've needed to reprint one.
Things might have been different had I committed to iPhoto's facial recognition piece. Having compared that with Google's (now unsupported, gee isn't that a familiar theme?) Picasa. Having done the work to identify faces on 20,000 photos, and having learned that there's no migration tool available to move from Picasa to iPhoto or Photo (****, not even to Google's Photos - talk about betrayal!), I am committed to the status quo. Like léonie, I'll keep Picasa, Windows 10, and my last paid for version of Photoshop CS5 on a dedicated PC that will never again be connected to the internet. (no potential updates to sabotage my tools).
Bottom line, I don't use my MacBook to manage my photo libraries, so I'm free to choose between Photo an iPhoto or whatever replacement Apple comes up with next - as long as the tools still provide some measure of functionality (like still being able to export a .pdf file).
As far as iCloud integration is concerned, I'm old school. I'll manage my own data and backups, thank you very much - partly out of privacy concerns, partly because, as a former coder, I simply prefer to maintain control over, and partly because I've burned in the past when companies I invested time and effort into suddenly went belly-up. Promises of life time warranties don't mean squat when a company goes bankrupt. (Remember Music Match? How about Kodak Gallery? to name just two. I didn't think so...)
Don't get me wrong, I do love my MacBook Pro. I spent 3 times what I could have paid for any of a number of Windows based PCs, just because I think the Mac's touchpad is vastly superior to anyone else's. Still, I use the Mac primarily as my travel PC in Boot Camp running Windows 10 and a recent copy of my data on board.
Well enough dawdling and diversion, back to the calendar project at Shutterfly - only 4 months left to go. shouldn't take more than a week...