I asked all of our friends to upload to a Google Photos shared album. Most of my friends use Android and most of my wife's friends use Apple-- didn't hear any complaints about Google Photos. Our experience was that most people have a google account anyways and are familiar enough with Google Photos.

The people I asked for advice said that they'd prefer to download Google Photos to upload/view photos vs. a third-party service that didn't require them to download anything (i.e. Synology photo request).