You'll notice reading reviews here are very mixed, and the reason for this are those aforementioned small teams. While this creates agility, is also creates huge disparity. The manager pretty much controls everything, so score a bad, ineffective or incompetent one and you're life just became hell. Some teams have insane on-call demands, others crazy work hours and deliverable pressure, some dysfunctional processes and decision making, and yet two floors away there'll be a cluster of 100's with great work life balance, competent management and great processes. It really is hit and miss, and if you'e unlucky you're stuck for the next 12 months.
Management in general is also hit and miss. The general shortage in the sector has meant especially in Seattle a flood of ex-Microsoft jumping ship, bringing with them a mentality which is as alien to Amazon's original culture as you can get which is starting to show. Decision making is increasingly pushed up the chain by deadlocked committees that have way too many people present, project launches pushed back, QA blindly executing thousands of test cases which no one has any serious faith in but yet will still block releases, etc.
There is an element of shortest-path mentality to the customer focus (frugality after all is a core value). This isn't Apple - rarely is the "best possible experience" a factor in decision making, rather push it into production as quickly as possible and worry about coming back to fix it later, knowing full well that actually never happens. The experience is patchy - some areas of the software will be amazingly tuned while others barely function. Some teams won't do anything without data and an A|B test to back it up, where's others will agonize for months over the right color background for the top-right corner in meetings that drag on for hours way overloaded with senior management.
Benefits are pretty minimal. Sure there's great insurance, commuter benefits, etc. but food is minimal, catering is non-existent, free drinks or entertainment a rarity and leave entitlements stingy. The technology they give you to work with is a joke when 32Gb of RAM costs like $100 these days and laptops come with 4Gb and weigh a tonne. Watch out - your 401k dollar matching is in Amazon stock that won't vest for 3 years, your RSU is heavily weighted to your 3-4 years and most employees don't make it past 2. Pay rises are small and grudging, and bonuses pretty much non-existent. Leave entitlements are really stingy.
Most mobility is lateral - individuals moving between teams - as getting promoted is really difficult, even from SDE I to II. Most senior positions that open up are filled from the market, and career mentoring is pretty much non-existent. Promotions are done by committees with managers fighting each other for a small number of slots, regardless of the merit of each individual, and "bar raisers" who will actively oppose anyone not performing above a bell curve that is disproportionately out of sync with the reality of the actual talent pool. For senior engineers don't expect to move up the ladder past your initial hire level anytime soon.