Kuba described it really well, on top of that few small additions from my side.
One code:
- Tracker debugger flooded with traffic from many websites. Can be mitigated with filtering by Page URL (segments don’t work with tracker debugger)
- Custom dimensions limit - it’s 200 per scope but in case of a need of setting up a lot of them per domain, the limit has to be taken into account
Meta sites:
- Creating funnel, user flow, attribution reports is not possible. If you’re interested in tracking customer journey across many domains then one tracking code is the choice.
- More granular permission management (to each site separately)