"Nice enough," as in, "He's nice enough." The person being described as "nice enough" is usually never nice in any sense. This is a person who leaves you on read for hours only to respond, "Sorry, forgot," and not answer your original texts. It's a person who tells your crush you like her before you get the chance to. It's a person who is only nice when they feel like it.
The concept of "nice enough" just plays on the person speaking those words own kindness. They are the ones tolerating the erratic friendship in the hopes of those few nice moments.
This is a message to all those people who tolerate "nice enough": you deserve more. It is really hard to let go of people, but you are not giving up on them. Instead, you are recognizing that a friendship of "nice enough" gestures is not enough for you. This does not require you to suspend all communication with "nice enough," but it does require you to stop putting your entire heart into a relationship that eats away at your energy.
Approaching the person who is nice enough about your feelings can be a great solution so that the friendship may continue, but on more equal grounds. However, if "nice enough" does not change and continues to disregard you and your feelings, the relationship could be verging on unhealthy.
There are many solutions, and it just depends on what you are comfortable with: talk to them about it, slowly cut off communication, stay in their lives as a background player, stay in their lives but make it clear that you are not willing to allow manipulative or aggressive behavior, etc.
Nice always seems to be swallowing heavy doses of those people who are "nice enough," and sometimes that is OK. However, most of the time these people take advantage of your feelings and trusting nature. A dose of their nice side every once and awhile does not count as a friendship, and it is important to recognize what you get out of the relationship.
This is a PSA that you deserve more. You deserve a symbiotic friendship, and demanding this from your friends does not mean that you are being selfish, it means you are being realistic.