Complex features in stainless steel can indeed be CNC milled. However, it can be more difficult than simply milling stainless steel with basic features. This is so that precise machining and more specialized tools can be used to create intricate features. Here are some factors that can make CNC milling stainless steel with complex features more difficult:

• The complexity of the feature: The more complex a feature is, the more difficult it will be to machine. This is because complex features often require multiple, incredibly precise passes of the cutting tool.
• The size of the feature: A feature's machineability decreases with its size. This is because small features are more susceptible to being affected by chatter and vibration, which can lead to a poor surface finish.
• The material of the feature: Certain materials can be more difficult to machine than others. Because stainless steel is relatively difficult to machine compared to other materials, complex features may be more difficult to mill in stainless steel.

When CNC milling stainless steel with complex features, special tools and techniques are needed. Sharp end mills are designed specifically for this purpose and an exact and accurate CNC machine is required to mill stainless steel. You may also need to apply coolant or lubricant to prevent the part from overheating.