With all the patches, the mosaics are all easily obtainable now (no more doing horse gymnastics to get the one randomly above a cave in the Hinterlands, and the one in the wolf cave is now where it should be), if a bit time consuming. I made a list as I went of where I found them, then discovered I pretty much remembered where they all were without the list, though it has been helpful in trying to explain to other people where they are. 😛 Each mosaic is either located in one map, or in a certain group of maps (like the Dales), except one (which is still only spread out over a few maps). Most also require you to do several other sidequests to get all the bits (astariums, the tombs in Hissing Wastes, killing dragons, etc).
I like the Astariums too, especially in JoH, those seemed a bit more difficult (or maybe it's just because I've only done them a few times compared to the others). Bit sad they didn't put new ones in the other DLCs. 😛
The only collection I've never completed in any playthrough is the bottles one. Probably get to around the same number found in every playthrough (all 14 or so of them... yeah, I played this game way too much), so I don't know which ones I keep missing. 😛