I loaded up Stylish to my homepage and waited a few minutes while watching the task manager. These are the results.

21,724
196,868 (High 243,542 - Low 183,516)
160,984 (High 210,869 - Low 146,704)
77,268

This is Stylus below, and yes I restarted the browser.

23,696
68,744
It kept jumping, constantly, between 241k and 401k it never settled for even a full second. (High 692,152 - Low 224,526)
Same here though not as bad, 131k-134k. (High 233,362 - Low 130,909)

Also at least on Fanfiction.net when viewing stories that have long chapters Stylus doesn't do smooth scrolling. It all goes black and then after a few moments loads the text as you go towards the bottom. Stylish doesn't have that problem. I also only have a single Style loaded up on Stylus while Stylish has three. Both have Global dark style - change everything to DARK. Stylish also has Youtube Umbra on and Fanfiction.net in DeepDark Style.

Turning off the one for Fanfiction resulted it Stylish still not having the problem with loading up long chapters like Stylus has. I know there are supposed to be improvements, but memory usage and stability isn't one of them. Yet. Hopefully this gets improvements and can one day be as good if not better than Stylish.