Regarding energy, people are talking about 120 free energy per day, but there's a lot more than that. There's 3x45 free energy + 1 energy every 6mins = 375 energy per day.
While I don't doubt that a fair amount of that will be budgeted for farming hard nodes, that is a false equivalence: you should then compare buying shards in shipments as well, but the equation gets needlessly complex.
Mk V furnaces are always a good buy. At 1400 for 120 purple salvages they are almost a steal.
The rest don't seem worth it. Even without the free energy, you barely break even, and spending some of the free energy, it's always more expensive in the shipment.
As for time spent: unless you are getting more crystals than you can spend on cheap refreshes (iow: > 400 crystals per day), arena wins out in terms of time too.