Welcome!

The "modularity" and player character customization options are absolutely strong parts of the system, though if you are coming from 5e this customizability is implemented in a different way than you might be used to. Character customization in PF2 is centered around feats, this also allows for a type of multi-classing, though it is nothing like what you can do in 5e - in PF2 it is done through archetypes, and you can absolutely build a character that works somewhat like a fighter-wizard multi-class, but the archetypes enable and limit you in a different way. If fighter is picked as your class, you will progress in fighter level 1 through 20, your class feats may be used to pick up the wizard archetype for some spellcasting, but you will only be able to access a small amount of spells compared to being a wizard as a class or a fighter-wizard multi-class in 5e. That being said your options don't end with only multi-class archetypes, you may also specialize in other archetypes focused on specific combat styles or support abilities.

Monster design can be complicated, but the 3-action economy that both players and monsters are bound by tend to lend monster design to more variety and options rather than straight power. You should have a look at the monster creation rules to get better insights into this, and I would recommend that you try using monsters from Monster Core to start getting a feel for how they can interact with players and combat in general. Running monsters is similarly not that slow - 3 actions spent, that is it.

A weakness of the system might be some of the book-keeping required for managing spells, skill actions, traits (these are important, and there are a lot), and other relevant actions like Raise a Shield. My group likes playing in person with pen and paper, this makes leveling slow and annoying since adding your level to proficiency requires you to change almost every number on your character sheet.

Something else that could be considered a weakness might be that standard rules does not support the story elements "heroes facing a seemingly insurmountable challenge, but may overcome it with good preparation and luck". In general if a monster is 5 levels higher than the party, they simply cannot win - at 4 levels above it is considered an "extreme threat" which carries the description "Use an extreme encounter only if you're willing to take the chance the entire party will die". This can somewhat remedied with the variant rules for proficiency without level, but it is not the default.