ABAG was once a good agency for doing comprehensive planning studies. Like all regional planning agencies, it had and has no power to compel people to follow those plans; actual regulatory land use planning in Calif. is at the local level except for things like the Coastal Commission and BCDC. ABAG's board has traditionally been made up of county supervisors and city councilpeople, as are the governing boards of Metropolitan Planning Organizations (MPOs).

There is one semi-regulatory thing that ABAG has been doing: periodic housing responsibility allocations to local agencies required by the state housing law, which the locals then have to use or explain why they don't in their General Plan Housing Element updates every 5 years. In all other parts of the state where that is done (there's a lower limit on the population iirc) that allocation work is done by the MPO which otherwise is tasked primarily with regional transportation planning, funding allocation to transportation projects, and regional air quality modeling of the plans and funding programs. The Bay Area, for historical reasons, separate the transportation and housing functions; all other parts of CA have them combined in the MPO.

Also, MPOs aren't everywhere. They're a federal transportation law creation for "metropolitan" areas, usually but not always based on county boundaries (MTC includes 9 counties). There's a lower limit on population for MPOs (used to be 50K in an "urbanized" area, but probably higher now), and a couple of counties in the San Joaquin Valley didn't become MPOs until the noughties. Non-MPO counties in CA have state-law creations called Regional Transportation Planning Agencies (RTPAs) that do many of the same things as MPOs on a smaller scale, but don't have the intimate federal involvement the MPOs do.

SANDAG appears to do more planning implementation beyond transportation than the other MPOs. That's probably because of its close connection with San Diego City and County planning - while it's not the Planning Department, it supports a lot of what the Planning Departments do. Still, if a city doesn't want to do what "the plan" says it should, when push comes to shove their main way to "punish" somebody is with loss of federal transportation funding; but a small city may not care if that happens - they're mainly spending state and local money on pothole patching. That's pretty much the extent of the authority any of them has, though obviously the amount of money running around in the transportation game, even in the current restricted funding environment, involves a lot of zeros after the significant digits, and money *is* power.