What's really special about Jets in particular is it lets you have your cake and eat it too, in that locally you have what feels like a pretty normal monolithic Rails app, but when you deploy every endpoint magically becomes its own lambda...
OC also mentions that they run a job to keep the lambdas warm. One disadvantage with the lambda per endpoint is that he has to keep dozens of lambdas warm instead of just one.
Less "stuff" in each route, less boot/warmup time for each lambda.