Here is what you could do for a test: set up Limited-Z to do the amount of gain reduction that you want to have. Select one of the modes in the selection menu, for transparent style limiting try "Smooth", "Limited 2" or "Clean". You say it's breaking at some point, so make sure to stay within a reasonable range. Have oversampling engaged (4x should be fine). Now play with the x/y controls to refine the character of limiting to your liking. The different settings in the x/y control are basically different combinations of attack and release settings (fast attack / fast release, fast attack / slow release, slow attack / fast release etcetera) which you can also blend. So somehwere in the x/y field there should be a setting which suits your material. Make small adjustments to the setting.
Now compare the amount and quality of limiting you get with the others you mentioned.
Obviously, there still might be differences. For example the Tokyo Dawn Labs Limiter 6 has additional modules. To make a fair comparison you would actually have to use a mastering compressor in front of Limited-Z, since Limiter 6 has a mastering compressor on board. Obviously a pre-compressed signal is much less of a challenge for the actual limiting stage, than a highly dynamic signal. You could actually use Tokyo Dawn's Kotelnikov in front of Limited-Z if you want to compare Limited-Zs performance with TDL Limiter 6 with the engaged compression module.
Even if the actual limiting algorithm of Limited-Z indeed started "breaking up" earlier, I doubt that this would be in the range of multiple dbs difference in gain reduction. It's more likely that either Limited-Z is not set up correctly or there are crucial procession elements missing in the mastering chain before the limiter (eg buss compression).
Also, there is the general issue of people wanting "hard limiting" (squashed, punchy sound) and at the same time they want it to sound transparent. Those two characteristics actually mutually exclude one another. I mean, yes, there are differences in the way different compressors / limiters squash and saturate a sound source and one can sound better than the other or get louder. But generally, squashed / punchy sound is the opposite of "natural" and "transparent".
