About MacOS High Sierra Update and issues

I have submitted my MacBook Pro (early 2015) in Apple Authorised service centre since Genius Bar is not available in my country (India) for following issues

1. Not shutting down

2. sometimes apps not opening / not responding

3. Stuck in next (10.13.0) update

4. Every app needs force quit

5. app suddenly close in between work

6. Too much time for processing downloaded file

7. Safari not opening properly

8. File missing in iTunes

I have erased my HHD before submitting
but they charged me Rs.3000 and installed MacOS Sierra 10.12.6

I asked for 10.13.0 (High Sierra) since I'm using iPhone 7 on iOS 11.2 which requires APFS file system, but Engineers of Apple Authorised service centre say, "please stay on 10.12.6, Don't upgrade to 10.13.0, If you upgrade above issues of lagging will start again"

But my MacBook Pro (early 2015) is compatible with 10.13.0 than why should I stay on 10.12 ? Why issues will be back after update ? If I stayed on 10.12 than how can I able to sync my iPhone having iOS 11 ?