IBM AIX Kernel parameter limitations

Hi team! We have been a die hard HP shop for a long time, but we are about to start buying IBM AIX machines. I know I have heard complaints in some classes from other users of this platform because they were unable to tune parameters that INFORMIX told them needed to be increased. Can anyone give me a specific example of a situation that occurred when the environment could not be optimized due to the lack of tunable Kernel parameters? I know one user who was complaining because of problems archiving blob spaces. What parameters are not tunable in IBM AIX that are tunable in HP? Are ANY Kernel parameters tunable in IBM AIX? Thanks for your responses!