trimforce enable does not work
Solution 1:
I'm using a Samsung 850 EVO in my MacBook Pro. If you prefer a method that uses the GUI, I recommend Trim Enabler (it's $9.99, works well, and supports the EVO 850):
You also asked why they mentioned that trimforce might not be safe. This article in their FAQ might answer your question: Why should I not use Terminal commands like trimforce?