The FBF animation I typically need in my character work is usually more fx-related, like tears, splashes, fire and smoke, etc. This kind of stuff is easy to do with Moho's FBF--in fact, I used it at work yesterday for a quick 'sweat' effect. But, IMO, the Freehand tool is still a little glitchy and you probably won't want to do a lot of complicated FBF animation with it.
At work we also have Toon Boom Harmony and I'll sometimes use the FBF tools there because the drawing tools are arguably better (the
drawing part itself is fine but the UI is a bit scattered and clunky.) I prefer Moho for rigged animation so what I might do is bring my Moho renders into the program and draw on top of the renders. But, to be honest, most of the time, it's easier to do this sort of thing directly in Moho even with its current FBF tools. IMO, Harmony may be worth it if you intend to do a lot of FBF animation, but if you're just looking for something to supplement Moho's toolset, it's a bit expensive to get as an 'add-on'.
A couple of years ago, I got a license of
CACANi. It seems really good and it's relatively inexpensive ($15/month or $300 for a perpetual license,) and there's a trial version you can download. But I'm embarrassed to say I haven't
really used it yet. I'm hoping to spend more time with it when I finish my current personal animation project, and I'll post more info when I have more experience with it. I know a few users here also have CACANi so maybe they can provide more info about it.
Some Moho users here really like
TV Paint. I have an older license (I think I stopped upgrading at v.8 or 9) and have used it only a few times. It has terrific natural media brushes and it's great for getting that 'hand crafted' animation look. It's been several years but, when I used it, it was primarily a bitmap animation program, which may or may not be suitable for your needs. I'm not sure that it has vector drawing yet but obviously artists are getting some nice 'clean' looks from it too. (Maybe a current user can provide better info.)
Sorry I don't have any firm recommendations. My wish is that the Moho developers will continue to give FBF another round of improvements in upcoming releases. I think they got a good start with the current FBF system--it's fine for simple stuff but still needs improvement for more ambitious FBF needs. I will certainly use Moho's FBF a lot more when they do.