The ecosystem argument pertains to Safari because Apple doesn't produce versions of Safari for other platforms which means bookmarks and extensions can be shared across platforms as they can with Chrome and Firefox.

For Photos the argument pertains because it doesn't tie in with any other photo storage services except their own. For instance macOS has my Google account information and the ability to use it for Mail, Contacts, Calendars and Notes but Photos won't use Google Photos to store web albums.

Safari is an app using WebKit which means if WebKit is slow to adopt than Safari which uses it is also. Some of the technologies I care about that Safari/WebKit has been slow to deploy include WebRTC, a variety of standards around progressive web apps such as service workers and the Push API and support of open audio and video codecs.