Transmission level SCADA systems must be on their own private networks per NERC standards so for someone to hack into a transmission SCADA they would have to physically plug in at a substation to hack their way in or break into a control room either way at that point someone would know they are there. It would be easier at the distribution level that doesn't follow NERC standards but then it would be isolated to a smaller region and then what is the real danger if they command distribution breakers to open? Linemen dispatch, turn the controls to local, and close the breakers back in. But also there's like 3000 different distribution companies in the US so to cause a lot of trouble at that level would take a lot of coordination and breaking into a lot of systems. Yeah they could shut off a city and that would suck especially of it's really hot or any cold but it could be brought back up fairly quickly.
At the transmission level all they could really do is trip open lines which would suck but what is the danger of physical damage occurring? Maybe some other lines would be overloaded to the point of self destruction but in order for that to happen all of the automatic relaying would have to be disabled at the same time to keep the overloaded facilities energized until they fell down and you can't do that with SCADA. So again if your SCADA is compromised you dispatch techs to switch the devices to local control and close them back in. Would take a couple days maybe to bring everything back up and it would be suck but would not be catastrophic. And again it is one transmission operator at a time unless a huge coordinated effort is underway.
Generation plants, to gain access to controls in any ones of decent size you'd have to break in to the control room or the plant yard to gain control of the devices. The plants were tied to have their own control rooms on site.
A large EMP would fuck our shit up in a hurry though.
But bang for your buck, most cost effective way to fuck up a power system would be physical sabotage. A few 30.06 rounds from a couple hundred yards into a large transformer is a bad day all around. Then if you got a couple buddies doing it too at the same time then a lot of people in that region are going to be living like @Mooseoutfront for a while. Especially right now, most companies are running on very very thin inventory. I mean normally to replace a large transformer takes a while anyway but right now nobody has any spares and replacements are years out.
Anyway, if you require electricity to live then buy a generator and keep plenty of fuel on hand.