Fees tend to be somewhat higher for Niche ETFs. More creative strategies command higher fees. Also, if a niche ETF is actively managed or has an active management overlay it can contribute to higher expense ratios.
In some cases the fees can actually be lower because the Niche ETF operates in areas of the market where the competition is less intense. Fund size can also play a significant role. Higher asset funds also tend to be cheaper because they have lower overall operating costs.