Idk the technical details but the main ones I’ve heard are that it increases the hardware overhead of miners who want to be profitable, further centralizing mining, and creating MEV. The idea is that widely used sidechains would result in miners needing to run nodes for them while also giving them more opportunities for out-of-band payments (not just network fees).
Discussion
My cursory understanding is that bitcoin miners can ignore sidechains but when a sidechain makes a synchronization transaction on bitcoin, the mining fee paid may be huge by including a percentage of all the mined fees of the side chain for that period. Although that sounds unnecessary, and more like a "drivechain good for miner" sales pitch 🚩