Unfortunately, for Bitcoin , there are some scenarios where there is only a cost in effort to perform an attack.
2 compromised mining pools could perform the attack right now.
Instant in the sense that 2-3 double spends would occur before being caught.
Bitcoin has its on set of security problems. Still more secure than any other coin but insecure nonetheless.
When judging the security of a coin there are many other aspects to consider such as the amount of peer review, the amount of nodes,
the amount of implementations and stacks, the amount of researchers auditing, the amount of developers, the total market cap which
incentivizes would be attackers from exploiting the currency, ect....