Just thought I'd report back here in case anyone else goes to Yllas to ski.
We only skied Yllas, because we figured there wasn't a huge difference between Yllas and Levi-- and Yllas wasn't spectacular, so we didn't want to drive.
The mountain itself had a decent variety of trails, but they were almost all super hard pack (but not icy at all), mostly pretty wide (with one massively wide groomer), and pretty boring. The blacks could have been difficult greens (in U.S. system), and the reds/blacks had nothing to distinguish them.
Lift-served off-piste was pretty mediocre. The stuff between trails were fine and fun, but still hard pack, and skied out like mad. Getting outside the resort (on lift-served, non-hike) stuff that would end back at the lift/road was also pretty bad. Too much traversing, tons of wild-arctic crust, moon-like windblow ice-snow, with razor edges and craters. I nearly killed the two boarders who tried to follow me on my skies. In great conditions, fresh snow (that stays), the off-piste could be OK for a beginner.
We had two big snow days... but the mountain was as naked (hard) as before. The wind was so strong that anything that fell was instantly transported off the back of the mountain, and deposited in icy channels. In fact, on the second day, the wind was so strong that I got blown UP a slow backwards... and it took me 20 minutes to travel what should have taken 30 seconds at most. I literally had to push push push, tuck tuck tuck. If I stood up, I went uphill. That's a first.
Lots of pull-lifts, one slow chair, one gondola.
The upside was the mountain was mostly empty, there was zero ice.
In other words: Fine for a day or two if you can't be in the snow for a week without doing some downhill (that's me), but come here for the Northern Lights, Reindeer eating, Sauna, and other outdoor adventures (really spectacular, I loved it).... not the downhill skiing.