One thing which will hopefully be considered an improvement, at least in terms of flexibility, is the ability to use the ROM of the SIDE/SIDE2 cartridge alongside the U1MB PBI hard disk. This will be at the expense of the ATR swap button (which still works just the same when the external cartridge is turned off), but will allow the use of the previously redundant 256KB of banked cartridge ROM while the hard disk is in use. Not that there's much to put there yet, but hopefully some conversions will be forthcoming. We'll see.
Anyway: the only thing that worries me is that having the ATR Swap Button 'Enabled' is now ESSENTIAL to preventing the SIDE cartridge's loader from booting instead of U1MB's SDX ROM or a disk-based DOS on a partition. Leaving the swap button disabled has never previously served any practical purpose whatsoever, since the external cart ROM was always suppressed by the PBI HDD regardless. But now that disabling the button allows whatever is on the cart to boot while the HDD is active, I worry about people becoming hopelessly confused by the fact they must enable the ATR Swap Button in order to stop the loader on the SIDE cart from booting instead of SDX or an attached disk or HDD volume.
I do not have a SIDE2 cart, nor do I plan to get one, so none of this has much effect on my system. What I do wonder, though, is if the ATR SWAP changes will have any adverse effects on UNO Cart or Ultimate Cartridge operations?