A risk is of course that your public environment is so much bigger than your biggest private install, that it doesn't make sense anymore to keep the same technology for both tiers. I think both PWS and gitlab.com suffer from this.
If so, they could (in theory) split Gitlab.com into a bunch shards which as a whole match the properties of N% of enterprise users. That'd be a pretty cool way to avoid the different-in-scale problem (although you might still run into novel problems as you're now the Gitlab instance with the most shards..).
In general, over time, load on some shards will increase while others decrease. Migrating a customer from one shard to another will likley cause a short outage for them, and many bugs down the line when they've bookmarked all kinds of things.