Soylent-like meal replacements are safe but they aren't necessarily healthy. Safe has a specific meaning in the food/drug world in that it means the product will not kill or harm you if used as directed.

Meal-replacement shakes can also be also effective if used properly for specific purposes (i.e, weight loss, muscle gain, etc.).

You are probably trying to see if they are healthy. Food is too complex to be reduced to a simple shake. But for some people, like those with diabetes or Cronin's disease, meal replacement shakes can be healthier than the alternatives.