For me no significant downsides as, as a Contractor, I would want to be as back to back as possible. Having said this, I think the biggest legal risk is often ignored by lawyers (believe it or not) and that is the risk (almost certainty) of not operating the contract as written leading to breaches of contract.
So while lawyers might want an full ECS as it is back-to-back legal with the ECC, the reality is that for a small subcontract, neither the Contractor or Subcontractor will operate it as written, so both will be in breach at some point. And if both a re consistently in breach, then it can lead to a big ball of spaghetti to unwind. So for smaller subcontracts, I suggest using the short form.