Smart Bones can only rotate in two directions, thus the limit. However...
I suppose you probably want to link an Action the the length or position of a bone. While Moho doesn't currently support this in its standard Smart Bones workflow, there are many tricks available to work around the 2 Actions limit.
For example, you can use target bones to track the length of another bone. If the target bone is parented so that its position and angle is relative to the bone tip it's tracking, you can add another action or two there. This also works for position. This is the idea that drives the many 'joystick' controls you see out there.
If you don't want to see all the extra bones that you probably don't wish to animate directly, use the Shy Bone option. This is a toggle that allows you to quickly show and hide bones that use it. (Tip: my MQC panel has a button that shows and hides all Shy Bones.)
You can also daisy chain Smart Bone Actions so that 1 bone may drive multiple Smart Bones. If you're clever with the hierarchy, you can control many actions with a few controls, and you can even have controls that operate different sets of Smart Bones depending on how you turn them.
All of the above techniques are done using only native features in Moho and you can do a awful lot with it. Most of the rigs I've build and animated in the TV programs where I work use only native Moho features, and animating the rigs is a breeze.
If you want more though, you might extend these capabilities by adding a script in the process. There are many useful scripts here you can try...
https://mohoscripts.com/
Look under the categories Bones, Smart Bones, and Actions. Lots of good stuff there.
Hope this helps.