The common beginner mistake is building a robot arm out of hobby servos and expecting industrial-arm behavior. SG90-style servos are fine for demos, but they flex, drift and burn out under real loads. A useful desktop arm starts with NEMA 17 steppers on the shoulder and elbow, small metal-geared servos only where loads are low, and belt reduction so the joints have torque without needing giant motors.
A practical first build is 5 axes: base rotation, shoulder, elbow, wrist pitch and gripper. Skip wrist roll until the arm can reliably pick up a 100g object. The frame can be 3D printed in PETG or cut from 5mm acrylic, but the shoulder should have metal side plates if you want repeatability. Expect a $140-280 BOM depending on how much aluminum hardware you use.
Control can stay simple at first: inverse kinematics on an ESP32, step/dir drivers for the steppers, and a browser UI with sliders. Once the mechanics work, you can bridge it to ROS2 using micro-ROS or a serial protocol. RoboHub can generate the first-pass CAD and firmware, but calibration is what makes the arm feel real.