Deflation is bad for creditors for the same reason it is bad for debtors. By making it harder to repay a loan, deflation raises the risk of default i.e. it makes issuing a loan riskier. Out-of-control deflation can lead to disasters where large numbers of loans go into default, ultimately bankrupting creditors.

"deflation and inflation are only damaging when they occur unpredictably. Predictable amounts of either can easily be built into the interest rate of the transaction."

The problem here is that as the economy expands, the rate of deflation increases if no new currency can be added to the system (as is the case with Bitcoin). As the economy contracts, issuing a loan is risky due to the economic contraction. Expansions and contractions of an economy are basically unpredictable; there are a few vague patterns here and there, but in general predicting economic booms and busts is difficult (if you could do it, you would be very rich very fast).

Issuing a loan in a deflationary currency is asking for trouble.