It's great that you brought up the size of the UTXOs. When you send just 2 UTXOs to the wallet for a CoinJoin, you often end up with nearly 20 mixed UTXOs, most of which are of low value.

Consolidating all your coins after a CoinJoin can negate any benefits gained from the process. I suspect that many people are doing this.

Summary:

 - You obtain many small UTXOs.

 - Many users will consolidate their coins into a hardware wallet (HW).

 - When using the CoinJoin option directly on the HW, you don't know how much will obtain or how many UTXOs you will receive.

- You could always be mixing with the same users, as there is no need for new incoming liquidity to start a CoinJoin.

- Efficiency is low: sending two UTXOs can result in receiving almost 20, leading to higher future spending.

- Being a free service, you are not protected against Sybil attacks.