At creationkit.com there are links to some opals. Above the download link there is a caution that this type of file can put malicious files in your computer. Are those links safe to download?

Yes they are safe. These are object palettes. Its like a favorites list for placable items in the CK. Its also true that any downloaded file from the internet could be malicious.

The warning must be some plugin or your AV, cause the warning isn't there for me and isn't built into the page.

Yes, they're safe, likely a general warning about downloading things on the internet.

Are all safe. I used and made myself some of these kind of files. I must say: Long Life to OPAL, the best time saver in landscaping and cluttering.

I hope they help. I'm making a small area, just a crater with a garden in the center, 3 X3 fields surrounding it and 5X5 mountain ridges encircling it. I find landscaping very hard, trying to get enough variation that the terrain looks interesting and natural. I wish I could just select everything in some cell in Tamriel and just paste it without marking that cell as edited. Natural areas from caves to worldspaces are hard for me.

5x5 is not so hard to fill. OPAL can save your time very easy. You can set random rotation, sink and scale on every object. Also, if you made a multi selection from OPAL then a random item spawn when you click in the render view: excellent for foliage and small rocks.

If you plan to make an area bigger than 5x5 then you should consider to use region filling feature. It can fill a whole worldspace and gives brilliant results with small objects.

May be slightly unrelated, but for some reason using OPALs for an extended time crashes my CK about 15 minutes into using them. Anyone ever experience this?

thx for the hint, i had no idea these existed