(And I'm not even going to touch the frankly invidious IE comparison.)
I grant testing is an issue if you don't own Apple hardware, although there are free and inexpensive remote testing services which cover that need well, and I'm certainly not about to say that Safari doesn't pose any unique difficulties to the web developer. But so does every other browser.
Safari and WebKit has 100% support for ES6, which is more than any other browser - http://kangax.github.io/compat-table/es6/
I think what you mean is Safari doesn't support the standards you want it to support, some of which aren't actually full standards yet but candidates.
Common problems include items not syncing, syncing out of time order, and conflict resolution causing synced data to appear lost. Most devs that are really serious about sync tend to roll their own after experiencing enough issues to make iCloud not worth the effort.
This is entirely based on my experience and may not be universally true but I've heard enough devs repeat my own complaints to feel like it's not completely out for left field.
Safari is perfect for my use cases. Way better battery life, feels lighter than chrome, etc, etc.
What exactly in your opinion is worse about modern safari and maps that makes a worse experience for the user?