Etcd uses bboltdb as the underlying database. BoltDB is based on LMDB, what is used in OpenLDAP. Both essentially implement a hierarchical directory based approach. So in my limited understanding etcd seems like a more modern and an easier to main approach for running a secure user directory service. Is that true? What am i missing?

Ldap usually is build around its data. etcd does not as far as i can see, but that shouldn't stop you to write a plugin for sssd or pam that provides the information.