Restore of backups (created with XtraBackup/Maria Backup) into empty data directory is the only valid scenario where success may be guaranteed. In other words if data directory has leftover data/log files from old Server instance - various problems may occur.
Nevertheless, on practice restore into non-empty directory may easily succeed without any issues (despite being dangerous).
So far both xtrabackup and mariabackup 10.1 did generate empty log files during --prepare.
Thus if user does simple 'copy' of prepared backup into non-empty data directory - old logs would be most likely just overwritten.
Change in behavior:
Mariabackup 10.2.7 doesn't create empty log files and relies on --copy-back action executed by user. (which will delete old innodb log files, if any).
For situations where users don't follow mentioned recommendations (i.e. neither use --copy-back or make sure that data directory is empty before restore) - backups created mariabackup 10.2.7 are definitely expected to become inconsistent/corrupted. (because of presence of old leftover InnoDB logs).