Denmark is virtually always placed on the bottom tier of any Tier List you can find here, and for good reason: Basically all of their benefits are towards domination, and of the domination-focused civs, every other one does it better:
Zulu get great promotions fast and are cheap to maintain, plus the Impi get two attacks.
Assyria have arguably the best siege unit in the game and multiple ways to make up any science/culture gap left by beelining their needed military strategies.
Huns have the other great siege unit in the game and can actually get it on turn 1 or 2 if they're lucky (from hitting the right ruin), plus the Horse Archer is crazy powerful itself.
Mongolia's UUs are so strong and synergistic that it doesn't matter that their UA is practically useless.
Spain's slot-machine UA can lead to an early win-mode and while their UUs aren't anything to write home about, the Conquistador's settling ability is neat for the play-style.
Aztec's UA + Jaguar can make them an early runaway very easily for just about any victory condition, not just domination.
England's control over the ocean, along with 2 of the best UUs in the game, speak for themselves.
Denmark just can't compete with that. It basically demands an archipelago map, and even there is outshone by both Carthage and Indonesia, neither of which are considered "good" by any means. Carthage and Indonesia both reward early expansion, though, which Denmark doesn't. Carthage isn't really built for domination (more like a weaker, roundabout route towards a science victory) but Indonesia is comparable to Denmark. Both want water-maps. Both rely on spamming UUs that require Iron. The Kris is available earlier than the Berserker and is better on balance, however (though this can definitely be debated.
The thing is, though... Denmark is really, truly fun. Not good, but fun.The invasion strategy is just a joy to pull off. I think of them as basically being like Captain Falcon in Smash Brothers. Is he good? No, of course not, by any tier list. Is it fun to land a falcon punch? Absolutely. So do it!