It's hard to quantify what is secure and what is not. Actually working out what the incentives are for the system is surprisingly tricky, miners have all sorts

It's hard to quantify what is secure and what is not. Actually working out what the incentives are for the system is surprisingly tricky, miners have all sorts of different motivations that conflict even when they would appear to be highly illogical on paper. Miners today include transactions with zero fees, where in a naive model of the network they probably wouldn't. The justification is often that it's financially sensible for them to do this because it inspires confidence in the system, making the price higher, which in turn makes their income higher. The lack of possibility for a pricing feedback loop makes this all a little tricky too, as the system can't make any changes based on external influences.