We had very rough seas from Valparisio to the Chilian fjiords, then mostly calm seas the rest of the way. The Straits of Magellan are inland and fairly protected, but you can get a lot of wind. The wild currents and tides there are not a real problem for modern cruise ships (as they were for the old sailing vessels). Cape Horn can be variable. We were lucky and although we had rain and large swells, (and it was COLD) at the horn, the night before was completely clear and cold and perfect for star-gazing (get out to see the Southern Cross and other unfamiliar stars!). The Falklands can also be rough (I guess about 1/2 the time too rough to tender) but we even had a calm and warm day there, perfect of a hike around the penquin sanctuary.
Luck of the draw with the seas and weather on this cruise, but you will have some great scenary and a good trip if you just prepare for everything and take the right clothing.