Ease of use is a big one. You can write your Webtask, run wt create and you'll get an endpoint to access your function vs Lambda's fairly complex system - especially if you just want to expose an endpoint to handle a particular use case.

The authentication/authorization is another big one. Auth0 is a lot easier to work with compared to Amazon IAM and Cognito.

Lambda does have it's strengths in the AWS ecosystem though and also supports multiple languages while Webtasks are mainly written in Node (and C# if you use a wrapper).

Disclaimer: I work at Auth0 as a Tech Writer and wrote the above article.