Writing a web part or web service may not be a big issue but deploying them is certainly a headache. You start getting permission errors as soon as you deploy your code on the server.
You will get following error
Request for the permission of type 'Microsoft.SharePoint.Security.SharePointPermission, Microsoft.SharePoint.Security, Version=220.127.116.11, Culture=neutral, PublicKeyToken=71e9bce111e9429c' failed.
There are three ways to assign execution permissions to your code:
1. Increase the trust level for the entire virtual server
2. Create a custom policy file for your assemblies
3. Install your assemblies in the GAC