It's possible that I let my fingers type there before thinking about it too much - and yes to continue to demonstrate my muddledness, I suspect it is different in some of these thematic ETFs early in the thematic. Let me be clear I haven't really done a lot of thinking about this and even less actual research. That research probably exists but if not would be an interesting project.
My suspicion is that a diversified portfolio usually of the larger, more liquid businesses early in a thematic likely does not even include several businesses that will be superstars in that thematic over time and that it is likely to include some big mistakes. Of course that's why we pay others with their big brains and research engines to do the research and construct the portfolio. My suspicion is that in these niche thematics, that's really really hard to do - they'll get the construction of the portfolio sub optimised at least as much as it is optimised.
My wondering is might it better with these niche thematics to get in a little later rather than earlier? What I actually do is if I think there's something there in the thematic is take a small position in my SMSF. Theoretically, this means I'll pay some attention to it and dig into the performance of the actual businesses and make some decisions. What it more usually actually means is that I just grumble into my beard about poor ETF construction.