Overused and Poorly Tuned

Alex's Mobs is the modern successor to an old mod called Mo' Creatures and carries its legacy for better and for worse. It's used as a filler animal mod when in fact it's a detailed and verbose ecosystem of mobs across many different themes. They spawn frequently enough by default to overwrite normal vanilla mobs and completely transform the nature of the game. They rely on one of the more egregious core dependencies out there, Citadel. And while many issues of Alex's Mobs can be fixed with lots of config tweaking, Modpack developers fail to do so, creating a frustrating and sometimes terrifying experience from nothing at all.

First off, Alex's Mobs comes with a breathtaking roster of almost 90 Mobs, one of which being a boss. Each mob is beautifully textured in a way that is genuinely faithful to Minecraft's style. They are also charmingly animated, my favorite being the catfish. When damaged, he will bulge his eye like a cartoon character. Another favorite of mine is the Comb Jelly, a rainbow jelly fish that drifts under the ice of cold oceans.

The thumbnail for this mod might fail to properly explain however that there's far more than a few farm animals in Alex's Mobs. There's a reason why it's not just Alex's Animals. Some of the creatures in here are the worst Modded Minecraft has to offer.

• The Crimson Mosquito is a Nether Mob that can "latch" to your face, blocking your view and ticking you for damage, staying put until you kill it. Did I mention it has the same amount of health as a Spider? In a dimension where stepping in the wrong place often means the end of your life, and sometimes your items, this jumps the list as one of the worst mobs to encounter.
• The Murmur is a cave mob that can extend its neck through and around over 45 blocks. Their head does not de-aggro if you warp away either, creating a horrifying scenario where you could /home out of a cave, only for the creature's head to hit through your floor. Like the Crimson Mosquito, it has way too much health, sitting just above an Evoker with 15 Hearts by default.
• The Cave Centipede is a mind numbingly horrific creature that is also in the caves. It has 17.5 hearts, is about 5 blocks long, moves faster than you can sprint, poisons on contact, and of course, it can climb walls.

For most of the aggressive mobs in this mod, they will attack you unless you are wearing a specific bit of gear or are under the influence of a particular potion effect. Everyone has a weakness or interaction, usually in a way that's secular to the mod by itself or the vanilla game. An anaconda can be pacified with Raw Chicken. A Tusklin will attack unless you feed it Brown Mushrooms. In addition many of these mobs prey or interact with other mobs within Alex's Mobs. An Anteater will indeed eat Leafcutter Ants. And the AI for many of these mobs can be downright advanced, such as the Cockroach, which will run from areas with a Light Level at or above 7. They consume dropped food, get eaten by Cave Centipedes, run from players, can be fed sugar and bread, and can live without their head. They even dance next to a Jukebox or wear a sombrero if given a Maraca.

There's two issues with these mobs being as in-depth as they are.

1. It creates an insular environment. Unless twisted in the way that Intergrated MC twists Create to function, the mobs in here react with the very world they are overwriting and nothing else. While not as aggregious as the Aether, locking you out of your equipment and asking you to start progression over from almost nothing for this one dimension, Modpack developers really need to put the work in to make the Mobs in Alex's Mobs bearable for any playthrough or modpack. I will echo other users here in saying that this mod would do well in a Zoo pack just because it's fantastic on its own and holds up its own experience. It's built to be the star of a modpack, not an accessory, but that's how it is used.

2. All this advanced AI comes at a performance cost. I mentioned it before, but Alex's Mobs spawns a LOT of these guys if using the default config. It's not unusual to see vanilla mobs rarely because Alex's take the mob cap for themselves. And when there's so many of these guys running around with AI that's as complex as early Sims titles, especially in a modpack that's already bloated with 800 structure mods and zombie expansions, it's a killer. There's very few mods that you could blame for boosting the RAM requirements of a pack but this is one of them.

Alex's Mobs isn't buggy, I've never crashed or seen something that was functioning abnormally. It isn't ugly, everything fits in its own weird and charming way. It isn't poorly designed, the creatures are clever and have interesting behaviors that one could grow to love.

But if you're a Modpack dev, you have no business using Alex's Mobs unless you know what you are doing or if the Mod dev updates the default config for Alex's Mobs to be more friendly with little tweaking.

In short,

Use Naturalist instead.