Hello Ron,
I think this very much depends on your definition of druggable and the mode of action of the drug in question, so I would be careful to generalize here.
Consider for example MLL-fusions, a well known oncogene in hematopoietic malignancies (nicely reviewed e.g. by Robert Slany PMID 26923329). There are several known fusion partners and 70 something breakpoints in the gene, so some fusions will retain more or less of the MLL protein than others. Just because a drug will target a specific MLL fusion doesn't mean it will target all.
Another example that comes to my mind is resistance to Imatinib treatment due to activating mutations in the tyrosine kinase c-Kit (see e.g Growney et al. PMID 15790786), so even in the case of just a single gene instead of a fusion it isn't guaranteed that a drug that nominally targets that gene will work for an individual patient.
Best
Matthias