The overnight buses Santiago to Mendoza are not year round as they coincide when the border is open 24 hours usually most of the year except the winter months.

Always remember how clogged the Chile-Argentina border can get due to closures because of storms and horrendous backups due of holiday travel, strikes, road construction and even events like the recent Copa America.

I recall that there is no non-stop service Santiago to BA as the trip is broken up with a stop or change of buses in Mendoza.

If you are traveling to SCL on a OW airline, you should take a look at LAN's SA Pass and fly SCL-MDZ, MDZ-BUE (and one more segment to qualify for the pass) which may be the same in total as first class service on the bus.