Although davidenos has fixed the 'iCloud tabs not syncing' issue by toggling on and off Safari in iCloud, I want to share my experience with two 'issues' which appeared to be a feature, not a bug:
1. Since I was confused that on EACH device - MacBook, iPad and both iPhones - safari tabs on a new start page did always display only ONE device, I discovered that tapping / clicking the device name at the top of the tab list reveals a dropdown list of ALL devices, from which the device can be selected, which safari tabs should be displayed. Apple just has uncluttered the start page to prevent overloading the start page! Even if this is not worth mentioning for most users, some might struggle because they've overlooked the arrows next to the devices' name like I did...
2. If Safari tabs do NOT show up at all, the feature might just have been deactivated in the start page settings. Just try the following steps: open a new tab, scroll down to the "edit" button, and check whether the "iCloud tabs" toggle on at the bottom of the edit menu is active or not! Right here, the start page can also be rearranged / customised, and applied to all devices or only the current one.
Hope this hints may help to avoid some useless workarounds!