Thanks to all-too-soon un-necessary respawns, nothing is very safe. I suppose the VERY simple concept of you taking over a dead man's home was too much to envision for the dev team. Or that a dungeon you cleared a week ago might remain unpopulated and un-junk-ified is just too unrealistic. it's more believable if Dwemer ruins get flooded with more ancient artifact-creatures on your behalf, or that more ancient Draugrs burst AGAIN from their final resting places that you've already seen.

Not too many containers are very "safe" thanks to stupidity.
But the two barrels outside of the smith in Whiterun stay safe for a very long time--as well as the laboratory containers in Blackreach.

Sorry if that didn't quite answer you...

That word "stupidity" goes through my mind too.

Seems third-rate programmers do most of the work.

Geniuos creative programmers seem not to be wanted anymore. They are around.

Breezehome's barrels are not all that safe stuck some junk in them that I did not care about to "test" them and sure enough after about a month of "in game time" the stuff was gone and the potatoes were back, everything else in breeze home is stable the chests, nightstand, and such I have had stuff in them for over a month of real life time and its all still there, same with the house in Riften I wouldn't trust the barrels or any of the houses that have barrels for that matter. The free room you get at the college is also safe my robes have been hanging in the wardrobe for as long as I can remember. Never tested the Thieves guild are but heck its filled with thieves would leave anything around them anyway

The Dawnstar Sanctuary is the only place I've waited thirty days to check if it was safe (thirty-one actually to be on the safe side).

Sure enough after a month long nap, my ancient nord arrows were still in every container in there.

Barrels and sacks aren't safe anywhere - they hold food items or ingredients and they always respawn (have done since Oblivion, can't remember in the prior games).