Just spent 7 months in the Caribbean in a new SO 42i. Met loads of other liveaboards in all sorts of boats. Would not trade mine for any of them - why?
Comfort - large, open, spacious, especially the cockpit which was the envy of all those who came on board. Loads of handholds absolutely no problem moving about in any sea and we had some bloody rough passages on occasions.
Manoevarability - we can steer backwards in Marinas!! and we don't have a bow thruster. Talk to an Island Packet owner how he goes stern to - the answer is with a lot of help from shore!!
Speed - bloody quick, I did Antigua to Guadaloupe at an average of 8.8 knots (GPS not log). When we left Grenada in with three Island Packets - we left 1 hour later and arrived 2.5 hours before them. Getting somewhere 25% faster than a traditional boat is worth a hell of a lot.
Sea Keeping - yes she does slam head into sea especially when motoring - so we don't go!! Thats the beauty of living aboard. But once sailing she is no worse than any other type I have sailed (37years) it all depends on the wave and swell type as much as the hull design.
Would I go further offshore? - we were parked in Nelsons Dockyard in Antigua for Christmas along with most of the Blue Water rally boats, many of which were of the same type -seemed to get across the Atlantic alright. We have a hankering for the Pacific in a year or two and I am not changing my boat.
So do I consider modern yacht designs uncomfortable? - no more so than any other boat. A traditional hull may be slightly more sea kindly in some sea states but I will have been in the bar with a large one chilling out, while he is still bouncing around. Meanwhile SWMBO and I can fully stretch out on either side of the cockpit and watch another brilliant sunset, rum in hand.