In Klopp's second season at Liverpool, they were inconsistent. They finished 4th.
Inconsistent means that they won games by 4-0, 4-1, 5-1, 6-1, could beat City, United, Arsenal, Spurs... but they could also lose random games, fail to score, concede 3, 4 goals, etc.
This was true in his first season, too. They won away at Villa 0-6, at Norwich 4-5, beat City 3-0 and 1-4... but they would lose random games, fail to score, etc. even more often.
The problem with United right now is that they are consistently bad. United have not won a single league or CL game by more than 1 goal. They concede in almost every match. They have lost every game against a good opponent.