I liked the DLC, I think mostly because I love Lore and the codexes provided a bit more depth to the story. For people who simply collect the codexes for the sake of XP or just for completion sake, this is probably where BioWare fell short. They should have put a lot more effort into conveying the information in the codex into world events e.g. The spirits you find on the island etc.
They had a lot of potential for 'flashbacks' which would have contributed far more to the story and added a lot more intrigue to it... It would have broken up the fetch quests more as well and made them feel more important rather than the snippets from each quest giver at the end.
Overall I did like the DLC, but it fell short for the above reasons... I reckon if they'd relied less on codex entries people would have enjoyed it more as they weren't going to miss anything.
...As for Gaider... He was an okay writer, I personally found some of his work jarring in terms of his novels. He also did focus too much on politics and it began to show - while I get that he's an advocate for LGBT (and a very firm one), it was beginning to feel to me that he was trying to shove it down my throat even though this probably wasn't his intention e.g. Iron Bull snapping at me for a seemingly harmless comment regarding Krem. Granted the problem faced by Krem is a very real one, but snapping at me for asking a question so that I understand better is hardly a way to endear yourself to me!
I'm all for progressive stories, but not at the expense of a story's integrity and changing certain characters for the sake of a fanbase, an unpopular opinion perhaps, but not everyone in the world is going to agree with each other or see eye to eye.
Hopefully Gaider will stay on board on an advisory level for the next projects though, he wrote and designed Cassandra and Dorian beautifully, Dorian is fantastic, as is Cassandra, and I wouldn't want to see them go the same direction that Anders did (which destroyed my favourite character from Awakenings in one fell swoop)... But I'd like to see what Weekes can do with the franchise now. Weekes has a lot of talent and he deserves the opportunity, a new pair of eyes and creative direction would be a healthy change too.