OTS is not enough you have to force the attacker to publish the fake checkpoint, otherwise there is no cost for the attack and the rational thing to do is to do it just in case the honest chain is not...
With rootstock, I guess the main discomfort really does come from the fact that its funding isn't endogenous, its being pegged to Bitcoin. Looking into this a bit further, it seems like the pegnatories aren't anonymous happily, but it's still unclear why one would want to trust a handful of companies with peg-outs. This feature makes it seem rather fragile.
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