Your CPU limits are given by the hardware. A typical BasicX-24 is a 7.37MHz CPU with 32K bytes of user program and data storage, has a maximum program length of 8,000 lines, and has 400 bytes of working RAM (providing a maximum executable program size). It has two serial ports (1200-460.8K baud) and 16 standard I/O pins that can also be used as 8 10-bit analog inputs (ADCs) or 8 digital I/Os. It runs at 5V.
An Arduino, depending on the variant, has 14 standard I/O pins, allowing for six to eight 10-bit analog inputs. The Arduino Mega has many more I/O pins, but it also draws more current than other Arduinos. Most Arduino variants use digital pins 0 and 1 for a serial line, but the Arduino Mega has four serial lines. The standard Arduino (Arduino Uno) has 31.5K of effective storage and 2K of RAM. Arduinos run at either 8 MHz (requires 3.3V) or 16 MHz (requires 5V).
A good satellite design will not push the limits of either processor for the simple reason that your payload must be designed to be minimal, stripped down, and very robust.