Not impossible to drill. Early on we would drill deep as possible with a horizontal dowel drill machine and finish with a long Electrican drill same size in a hand drill.
30 years ago we did a door job for Disney World that required 1” hole in stile ends for break away hardware. I don’t remember how deep,24” plus at least. Laid the drill press horizontally with a complex jig and drill extensions. It was hit and miss, ruined a few stiles in the process. Done before door assembly.
In the end we found out a inlayed groove would have been acceptable. Miscommunication between architect and job supervisor...