Answered User Friendly?

If you configure MODX well and provide sufficient training to your client they can operate a site independently of you. I have clients ranging in skill from competent to almost none all looking after their own sites.

There are a number of ways MODX can help you protect clients from themselves such as automatically detecting and resizing images that are too large.

However if MODX was configured poorly and supplied without documentation I'm sure they would struggle.

Sounds like you would benefit from setting up a test site to evaluate MODX to enable you to make the best choice for your client. And yourself you need to be happy with the learning curve and workflow involved.

Or give us some more detail on the project and type of content the client needs to be able to maintain and you may get some help specific to your situation.