Viral marketing relies on the audience doing the distribution. That makes it a property of the content rather than a channel: nobody buys virality, they buy a chance at it.
Physical installations have a real advantage here, because a screen or an experience in a place is photographable and location-specific. A striking anamorphic billboard travels further on social media than the same spend on social media would, precisely because it exists somewhere.
The caveat is that "make it go viral" is not a plan and treating it as one wastes budgets. The base rate is low, the outcome is largely outside your control, and campaigns that fail to spread usually have no fallback because distribution was assumed. Budget as though it will not travel, and treat the spread as upside.
The second caveat is that virality is content-shaped, not brand-shaped. Content optimised for sharing frequently reaches an audience with no relationship to the product, which is a large number and a small result.