Based on what they've invented, I think Brainiac 5 would do a better job at building Frankenstein’s inventions than vice versa. Brainiac 5 has a much higher level of intelligence, knowledge, and skill than Frankenstein, and he is familiar with advanced technology and alien biology. He could probably replicate Frankenstein’s experiments with more precision and efficiency, and avoid some of the ethical and moral dilemmas that Frankenstein faced. He might even improve on Frankenstein’s inventions, such as making the creature more stable and less monstrous.
Frankenstein, on the other hand, would have a hard time understanding and using Brainiac 5’s inventions, since they are far beyond his era and scope of science. He might be able to grasp the basic principles of some of them, such as the flight ring or the force field belt, but he would lack the materials and tools to recreate them. He would also be overwhelmed by the complexity and power of Computo, and he might unleash him without knowing how to control or stop him.