God no, not RSM!

Is there not a difference between winding up and winding down?

Yes, quite - but I don't know much about Lendy. The point is that Lendy is in administration and may be wound up as a company and RSM is there as the creditors' trained Vulture. In the case of Assetz, RSM is the current contracted service provider for the Standby Plan, required by FCA, within which is the option of an orderly wind down of the loan book - working in effect for the lenders. These are very different things. MT is winding down its retail loan book in a managed way. 

It does not necessarily follow that RSM would be appointed as Vulture if the current unpleasantness at Collateral, Lendy and FS were to be repeated here. However, those appointed Vulture all tend to behave as vultures must.

In any case, I am lending a substantial amount (for me) through Assetz. Ergo nothing can go wrong.

Unless the Platform is prepared to self wind up ala MT, I think wind up merchant can very quickly and easily turn into creditors vulture though blender, Baker Tilly Creditor Services LLP aka a wholly owned subsidiary of RSM Group were Lendy’s FCA approved wind up merchants

From Lendy’s T&C’s: “If the Lendy platform were to fail or Lendy and/or Saving Stream Security Holding become insolvent we would transfer our obligations under the Terms and the Loan Contract and the Finance Documents to Baker Tilly Creditor Services LLP a third party back up servicer, with whom we have entered into a back up servicing arrangement with a term which renews automatically each year.”

On their appointment as Administrators RSM stated “The administrators are working closely with the FCA who consented to their appointment over the Companies.”

As I think we are all finding out Platform's winding-up/becoming insolvent or ceasing to trade near guarantee additional investor losses. Platform “risk” is rapidly becoming one of the biggest factors in “Your capital is At Risk”. Something I am not sure many adequately factored in.

I've believed platform risk is a major factor for a long time - not that I knew what to do about it. One concern I have is that the remaining platforms seem to not understand where their vulnerabilities are. They seem to have a belief that 'it won't happen to them'.

But in fact all three defunct platforms are not in fact undergoing an orderly wind down, which is the weakness that I hadn't realised.  Instead they are in administration.  So if AC were to fail it's quite likely the same thing would happen, and RSM are in the front of the queue. 
In either circumstance you don't want them.  Firstly they keep saying they have sent emails which in fact never arrive.  They are a shower.  a complete shower.  Their performance so far is such that I suspect that they won't be in business that long.  They are just complete rubbish.  Capeesh?

Insolvency effectively trumps any "wind-down plan" to date and insolvency has an extremely narrow focus. This means that both legally and contractually all bets are off when it comes to the crunch and the devil is in the detail (and often investors weren't privy to any of it). 

Where you thought you had a binding agreement with a platform as per its terms and conditions.... suddenly you may not. 

Do the new "enhanced" wind-down plans released in December genuinely change anything whatsoever? Like most investors, I don't have the appropriate qualifications in insolvency law to really understand. So answers on a postcard... 

You say potato, I say onion. I was just saying that Assetz have not stated in www.assetzcapital.co.uk/wind-down-arrangements that they have nominated any contingent administrators for winding the business up, as said in the OP. But I do agree with much you have said about what might happen in practice. I'm not sure that a different 'wind down contractor' would behave any better as an administrator. It's like a blood bank choosing a pet vampire.

Anyway, it's not going to happen.