I don't trust them, personally. They weren't designed for fabric use, and so I don't know what chemicals are in that ink and what they do to cotton fiber over time.
I have another problem with them aside from that concern: I tried using them anyway just to see how they did and apparently I draw my lines really fast - the friction of the pen tip running over the fabric vanishes the lines on me as soon as I draw them! LOL
I had bought a 3-pack; I took them all to work and use them there. Handy in meetings when people are being wishy-washy and changing their mind on me, I can sigh and dramatically erase stuff I had just written down. LOL