In working with FuelPHP recently a few thing have come up.  The devs on #fuelphp on Freenode have been awesome at coming up with or accepting fixes.  Here are the problems with immediate workarounds and long term solutions.
Fuel won’t let you refer to a View_Model with a templating engine extension
While View::forge('hello/world.twig') works fine, ViewModel::forge('hello/world.twig') fails to find the ViewModel because of the “.twig” part.
Workaround is to instead explicitly set the view on your ViewModel with public $_view = 'world.twig';
Long term fix is coming in Fuel 1.2.1 maintenance release https://github.com/fuel/core/commit/c06b0c67c027ee17bafa59cb089bd6c5baee80ab
Fuel insists that controllers are in the global namespace and prefixed with “Controller_”
No workaround.
Long term fix coming in Fuel 1.3 that will allow you to namespace and prefix controllers as you desire (as you already can with views) via a config setting: https://github.com/fuel/core/commit/882e564de66dc3f4edf83181b8fa36a8a8b083cd
Fuel’s View_Smarty does not expose the “plugins_dir” option.
Workaround is to set the option anyway, direct on the Parser object in your controller(s).  The trouble is that accessing this object (and working out if it is Smarty-related) can be complicated.  The naive version is something like:
$this->view->parser()->addPluginsDir(APPPATH.DS.'pluginsdirectoryname');
But if you want to continue to support non-Smarty rendering engines and are using ViewModels and Controller_Template it ends up looking more like (don’t judge me!):
        if (is_object($this->canvas)) {
            $rc = new \ReflectionClass($this->canvas);
            if ($rc->hasMethod('parser')) {
                $parser = $this->canvas->parser();
            } else if ($rc->hasProperty('_view')) {
                $parser = $this->canvas->_view->parser();
            } else {
                throw new YokoException('Unable to initialize Smarty i18n plugin.');
            }
            $smartyParam = 'plugins_dir';
            $ourPluginsDir = APPPATH . '/smarty';
            $pluginsDir = array_merge($parser->$smartyParam, [$ourPluginsDir]);
            $parser->$smartyParam = $pluginsDir;
        }
The long term fix is coming in the 1.3 release of Fuel’s parser package https://github.com/fuel/parser/pull/48.  The devs were very good about accepting this patch which bodes well for future contributions to the project.