H&M has positioned itself as more of an "eco friendly" and "fair wages" player but as a fast fashion retailer, has had accusations of similar practices in the past.
SHEIN seems a little different due to the nature, scale, and shameless lying infused in their approach. Their actions are orders of magnitude more intense even the grey zone manufacturing practices of Forever 21/Zara. Goes without saying no fast fashion is "ethical" by any stretch, but SHEIN is on another level.
[1] https://time.com/6695469/temu-shein-de-minimis/ [2] https://archive.ph/XRs9C
Traditional retail - like H&M - has a markup of around factor 10 (meaning if something is 100 in the store, the factory/shipping company gets 10) - and supplier lead times of 3-6 months.
Shein has much lower markup and lead times of down to under a day.
Compared to Shein H&M is a luxury brand.
Regardless, more insightful information would be very welcome here rather than hyperbole.