Placebo. The battery life improvement claim is usually negligible or a dramatic let down.
The other thing is generalisation. Specific issues are a good reason to change yet no one ever seems to give proper reasons. A bit like rooting, the pros and cons are usually avoided for simple generalisation.
I always liked both HTC's GUI and Samsungs. There are sufficiently similar to be a negligible change from stock.
And why has a tiny bit of GUI personality given way to conformity?