For the most part I'd heartily recommend Alex's Mobs, since it adds a ton of biome-appropriate animals that fit the Minecraft art style that make the world feel much more alive.
I had it installed when I last played Minecraft over a year ago, but I didn't make it far enough to encounter its many overpowered hostile fantasy creatures and now they've been responsible for most of my deaths in my current world.
My first encounter with one of them was when I was scoping out an ocean on a boat and was suddenly being killed by a giant skeletal swordfish in a matter of seconds. These guys spawn near shipwrecks, have an insanely long aggro distance and can outrun your boat so you're basically screwed the moment you accidentally wander near a shipwreck you can't even see on the bottom of the ocean.
Then I built a Nether Portal and realized I hadn't yet even seen the new Nether biomes. I enjoyed the view for about 30 seconds until I was swarmed by giant mosquitos that attached to my face and sucked half my healthbar away in one go. I managed to kill a couple but they just kept spawning, instantly making a beeline for me the moment they appeared. Very fun and balanced 
After I'd gotten annoyed and just disabled the mosquitos from spawning I was building a bridge across a lava-filled chasm when a gigantic snake skeleton knocked me off and I burned to death. My first warning that they existed was the second one of them leaped up at me from a dozen blocks below. At least they're only limited to lava lakes, I guess.
Later, I was back in the Overworld exploring a giant deepslate cave when all of a sudden I was being attacked by a shrieking head floating up from the caverns below. After I once again got killed in a few hits I looked up what the hell even happened and it turns out it was this Japanese youkai thing that has an attack range of 50 fucking blocks. Just now I was surveying another deep cave from a ledge, saw one of these spawn in way below and it just aggroed instantly and murdered me.
I mostly like these monster designs but why do they have to be this overly aggressive and powerful? They give you no warning and can outrun you and kill you in seconds- it's just completely out of tune with the way Minecraft's hostile mobs are designed. Even the more dangerous mobs like Ghasts make their presence well known far in advance. I guess they're balanced for players kitted out in enchanted diamond armor but they completely kill exploration for me... until I disable them in the config, but I just wish you could turn their aggro range down instead.
The mod also adds several large dangerous animals, like crocodiles and grizzly bears, but you can just avoid those or run away if you too get too close and they feel much closer to how Minecraft usually operates.
