I'm a toonboom girl no contest.
I can't imagine why you would make your bg's in toonboom - Professionally and personally I've always used Photoshop for backgrounds and imported them into Toonboom as png's. If you want vector bgs you could make them in Toonboom then export a png and import that into your scene file. As for lag- I've had a 500 frame pan, cuttered over a still background with 3 fully rigged characters connected to 7 deformers - not a trace of lag - that's a computer issue.
Toonboom has a steep learning curve- and I've shown features to some of my co-workers who have been using it for nearly a decade that they didn't know were there. but it's so much more flexible and animation oriented than flash is. Flash is a web developer software which can be used for animation. Toonboom is an animation software, developed by and for animators.
for cut out, or cheap tv animation it's better because of the elaborate system of rigging and modules. The network isn't just for FX, but rigging, and selection can be done there easily. There's no way to create meaningful hierarchy in flash- aside from the dreadful nested symbol mess. You can also create expressions and link pegs in toonboom- so when I'm working as a breaker- I could create a train with 40 wheels that all rotate according to a single controller. The animator has full control without copying symbols or added work.
for classical work- the toonboom drawing tools are more flexible and pleasant to draw with, strait out of the box + way more settings and options if you want to get down into the complexities of it.
I could go on all day. As someone who makes a living using animation software, I would rather tear off my own skin than work in flash.
But I'm not bitter lol
- toonboom breaker, fx artists, animator, compositor / generalist