The whole issue is that, without computing first and follow sets, it's easy to make a mistake and think that the grammar is LL(1) when it actually isn't. When that happens, the if statement does you no good. It'll be deterministic, but will silently mask the ambiguity.

The only case I can think of is something like the dangling else problem or similar, which is very easy to detect.

You need to have a recursive call to the same rule or a "parent" rule, followed by an optional token match (note that this is already more than a simple branch on the current token since it is effectively a 1 token backtrack).

If there's any extra token match inbetween (like mandatory {}) you've already dodged the problem.

So I agree a mistake is possible, but "easy" I would not call it. It only appears in very specific conditions. The issue is way more prevalent in general PEGs.