I play on a PS3, and had no problems importing DLC from previous games. I have a lot of them, and I like to use materials and decoration from DLC when creating levels.

Question is: If some DLC is not working on PS4, but I use stuff from it in building a level, will PS4-users with DLC-importing-issues have problems playing that level?

Short answer: No.

The data for that DLC is still in the game, just some people aren't getting it in the transfer. Also, DLC is not required to play any level.

Basically, every DLC pack is added into the games files so that the game won't crash if someone joins in your pod with DLC you don't have. Same with Levels.