A data point, take for what it's worth:

I hung a hammock in my shed earlier this week. I was trying to get the angles right for a comfortable hang from fixed-height anchors, which temporarily required variable-length suspension. The tautline hitch has been a go-to of mine for a long time (since long before I slept in a hammock).

Using 7/64" Amsteel, I tied a tautline hitch on the foot end and eased in. Over the next couple of minutes, that knot gave all the way out in frightening 2-3" increments. I knew I wasn't going to hit the floor since the loop wasn't that large, but it didn't grip and took every inch of that loop until the knot was butted against my hammock CL.

I wouldn't try it again. It was a stopgap to get the length right until I could splice a small whoopie. On top of that, it was horribly bound up and really difficult to untie.