The 8 just before Ku Kuan was carved out of the cliffs. They did a temporary road (will be wiped out in any big storm) on the riverbed . But that can’t become a permanent road. They will have to carve out the cliff again. That is dangerous work using dynamite. Not the kind of road building they want to do in Taiwan anymore. Many dangerous sections of the Suao - Hualian road were done the same way. They have since made a lot of tunnels and those sections have been closed.
If the 8 is ever to open from Teh Chi Dam to Ku Kuan again, they will have to tunnel , rather then carve out cliffs. But as on the Southern Cross, the road is cut off right after IIRC the longest tunnel on it.
So even with tunnels there are still sections that are dangerous and prone to landslides when the next quake hits.