I've only played Fortress Defense and Genesis, and only a few Genesis forts at that.
Fortress Defense simply adds extra hostile races. If you add all the races and run a short world gen (less than 50 years) you'll end up with about a dozen hostile races at embark. You get invaded after about a year and then most seasons after that, usually multiple invading races at once. You'll probably lose a few forts to begin with then work out what's required. The main problem then is that it becomes fairly monotonous after three or four years. Every season starts with an attempt to clean up all the crap left by the previous sieges, then pulling the civilians inside in the last month of the season before invaders come again. The crap piles up until you have thousands of items of clothes and weapons and armor and body parts laying about eating into your frame rate. FPS death is then even faster than for a vanilla fort, unless you're able to use magma for cleaning purposes.
Genesis adds all kinds of stuff. There are different castes of dwarves, new animals and plants to grow, different kinds of humans and elves, orcs, and a few new enemies. Metal working is overhauled and there is a large range of new workshops, many aimed at training your dwarves in various skills. Animals are generally tougher, metals and gems more valuable. This is a broad mod that perhaps makes things a bit harder but mostly adds a lot of extra features to the game, both making it easier and harder.