anyone doing regular Unit reviews?

Xerte, on occasion. Usually for GLEX rather than JP Units. Else-wise, immediate Unit reviews once a Unit drops are up to the community. Generally those are a bunch of people in the thread knee-jerking to the Unit and comparing it to the immediate meta. If you're truly desperate for a second opinion's assessment, then go to Ushi. He has a habit of hyping things to hell and back, though.

Any sort of tier/ranking lists?

Also kind of went away. So many Units are the equivalent of a swiss army knife that it's less about whether or not a Unit is good so much as how well it works with others.

Except Regil. But only for his LS.

What Bundle Chests are worth it (if any), versus buying gems?

Ticket Bundles will always give more Summons worth than buying their equivalent amount of Gems. They're just far more restricted. It's to the point where one bundle converts 3 Gems -> 1 Ticket.

What currently active summons should I go for, if any?

Chuck a few at Guilty Gear, if only for Jack-O'. Otherwise, wait for a decent Resummon Gate with increased rates on Regil/Cleria/Shion.