ADS-B alone is insufficient for collision avoidance. In order to operate safely outside of special restricted airspace, drones would need to be able to visually see and avoid other aircraft.
ADS-B out is only mandated in an extremely limited subset of the airspace where drones will be likely flying (and then only after Jan 1, 2020).
ADS-B out is only mandated[1] in cylinders around class C and the 30nm ring around class B airports, in class A airspace (at/above 18000' MSL), and in class E airspace that is both above 10000' MSL and 2500' AGL.
In practice, that means that most of the volume of drone-navigable airspace does not require other aircraft to have ADS-B out transmitters.
In none of the airspace are other aircraft required to have ADS-B in receivers.
"See and avoid" is therefore both practically and legally[2] required.
[1] - 14 CFR § 91.225 - https://www.law.cornell.edu/cfr/text/14/91.225
[2] - 14 CFR § 91.113 - https://www.law.cornell.edu/cfr/text/14/91.113