Two different types of challenge:

Tanking- Encounter challenges/positioning/agro

Healing - Encounter challenges/dynamic challenge of healing avoidable damage (dps controls this challenge by avoiding or not)

Tanking Challenges are the most obvious when failed, but can almost all be learned. They are fairly static... but if you mess up- you get called out and harassed.

Healing challenges are dynamic and can't all be learned away, since it is variable, based on party performance... but if you do mess up, you have an alibi that the dps shares the blame since they should avoid better - tank dies and it's just your fault.

Both of these jobs have challenges. Personally, I think healing is the more challenging of the two, but tanking has the more severe social consequences of error.