Giant at least has the seatpost clamp covered. You can choose the amount of offset, 10mm or 23 mm, and you can angle the seat as you like. On my 2007, it was not the easiest seat post clamp to adjust, but you could set it up as you wanted, and once done it stayed where you put it.

I'm not advocating for ISP's here. There's a small advantage and some tradeoffs. I just think the downsides tend to be oversold, and I wouldn't rule out buying a bike like a Giant Propel Advanced SL, or TCR Advanced SL because of the ISP.

That said, I didn't replace my TCR Team Advanced with a bike with ISP.

Edit: after I posted, it dawned on me about your point on saddle rotation. If you want your seat off line, that would be an issue. However, I thinky you've got the same issue on any aero seatpost frame. I know you can't rotate the seat on my P2.