There's rarely a time where you don't make it on a bus - it may happen on the Springs stop if you're trying for rope-drop and are there during a very busy time - but I have stayed at Saratoga nearly every month of the year, and 4-5 times in a year, and haven't seen people turned away from a park bus during any later hours (but I admit I NEVER do rope-drop).
I actually prefer Congress Park for a few reasons - the walks to the restaurants and main building aren't too long IMO, plus you can easily drive over (park in the back by the boat dock and there's a back door entrance)...but I also like to sometimes use the Disney Springs busses after 11am and early afternoon to evening to get to some parks when I don't want to take the internal bus. It doesn't work for all parks as AK isn't easy to walk into, but for getting to Epcot I can take the Beach or Boardwalk bus, to get to MK I can take the Contemporary bus, and DHS the Boardwalk bus, and walk over...and it can be useful when leaving the park at closure after fireworks when the park bus stops are mobbed and it can sometimes be 2-3 busses before you get on - simply walking over to Beach Club from Epcot, or Contemporary from MK, and hopping the Disney Springs bus then just walking back home to Saratoga.