What is they story behind fiscal having unclear impact on YC

Fiscal deficit is financed by issue of gov bonds= but the text in CFA says the impact of it is unclear on yield curve but at the same time monetary policy have clear impact.

Someone please explain if YC is engineered, then what is the role of fiscal deficit in it?

How come fiscal deficit getting its finance fr issue of bonds to market have uncertain impact when it clearly seem to affect YC?