How to deal with incomplete Google Takeout downloads for Google Photos?

Question

Hello, everyone.
Greetings from Rio!
I have a huge amount
of photos and videos stored in Google Photos. And to not just keep
everything in the cloud, I've designated a large external hard drive
just to keep a local backup of all the images. And I started
downloading my entire collection. To do that, I called  Google
Takeout specifically for Google Photos and everything apparently
worked fine. However, when I opened the local zip files, I noticed
that there were very few photos and videos in each folder. And
several “.json” files referring to images and videos that were
not in the folder. It was a shocking disappointment. I was hoping
that with Google Takeout I could recover 100% of the photos and
videos stored in Google Photos, and also all photo descriptions,
geotagging and album structures, but apparently I was wrong. 
Could you tell me if
you experienced similar situations? If so, how did you proceed to
remedy the problem? Did you find a satisfactory solution?
Respectfully,
- Carlos

janvb · Accepted Answer

@ Carlos Alberto Teixeira
Yeah, Google Takeout for Photos creates a big mess for several reasons:
The program was originally written for Picasa Web Albums. At that time ALL photos were in albums created by the user. In Google Photos you can simply upload to the library, and Google seems to store the photos in day albums, which you can see under "Auto Backup" on https://get.google.com/albumarchive. 
Google Takeout was never adapted to the new situation: they just changed the label. That explains why the indication of number of albums in the Dashboard is wrong: it counts all these day albums.
As a result the download may be messy and contain duplicates:
Some photos are in albums you created, others are in day albums under "Auto Backup", and some are in both, which creates duplicates when you download.
When downloading a complete library, the .json files may be in a different batch than the corresponding .jpg.  
Thus even when it worked as intended, the result may be very messy and contain duplicates. I suggest to add ALL photos to albums in Google Photos (for example one per year - an album can contain 20.000 photos) and only download these albums. I never tried this for a whole library thus have no idea what you get when you select several albums. I tried for one at a time and that kind of works. You loose descriptions and other data added in Google Photos (they are in the .json files but not easy to recover), but you get both edited and original copies of the photos.
See this link for more detailed info about downloading.
Edit: there is  a work-around to find the number of items uploaded to Google Photos:
Go the Gmail on an Android device (sign in to your account)
Tap + to create a new mail
Tap the "Insert file" icon (paperclip) > Add file
Tap the main menu (left upper corner - 3 lines)
Scroll down and tap the Google Photos icon
On top it now says how many items there are in Google Photos

Carlos Alberto Teixeira · Accepted Answer

@janvb — My sincere thanks. You are a true oracle. Elegant, informative, precise and hopeful response, including a super informative link, and, above all, outlining a line of action that will certainly save me a lot of work in this mission to download my photos. A big and grateful hug. And my best energies for you and your family.
- Carlos

janvb · Accepted Answer

I added to my post a way to find out how many photos are uploaded to Google Photos. 
I checked on my account (+30.000 photos) by selecting them all (kept me busy for a while), and it turned out to be correct.