I read up on it for a while a few years back, but I never tried it cause I like chewing and variety.
It's important if you are talking about soylent as in the brand, soylent as in a soy and lentil based beverage, or soylent as a generic term for meal replacement beverages.
Some of these meal replacements are designed to just replace 1 meal a day, or 2 meals a day, etc, so you could develop a deficiency after a little bit. There's a DIY community to share recipes along with "nutritional completeness".
Everyone has different dietary needs, so even if a shake technically has all the nutrients you need, it might not have enough of everything unless you eat way more calories worth than you need. Humans are pretty adaptable, though.