Willian is a top quality footballer depending on how you want to use him. Applying stats on goals and assists (as important as these are) will never do justice to assessing a player like Willian. Such figures will make him seem useless. Maybe just like they will make an Iniesta look very ordinary (not saying Willian can be compared to Iniesta. I am just referring to the goals/assists stats. Legend has it that John Terry scored more or almost as many goals as Iniesta). What Willian brings to a team, more than any other quality is balance and link up play. He is a very hard-working, energetic, team focused and intelligent player. His managers have tended to use him not purely as an attacker but as the type of attacker you can use to do the hard work of defending, running around, playing the nuisance, linking defence with attack, creating spaces for others etc.
That's why I said it depends on how he is used and what a manager is looking for.
This is the context the Willian debate should be placed. For me, it all comes down to what Jose wants or is planning. In the season when Willian scored very few goals and had few assists, he was still one of Jose's most important players. Why? Because he did certain jobs for the team as instructed by his manager.
So while he will not dazzle or score many goals, I think he can still offers a lot (again, depending on the manager's plan).