The set of people capable of building one of these machines and incapable of evaluating the risks of doing so is nearly empty. I don't think that's a real significant concern, to the extent it is it can be adequately mitigated by slapping some warnings on the blueprints instead of asking people not to try and design things.
This effort may well save no one in this crisis. It could still benefit by making future ventilators cheaper, serving as prior art on bullshit patents that people try to get on basic components of a ventilator in the future, and so on. This will very likely allow the health care system to funnel money into more effective life saving efforts in the future.
Think back to Wuhan. Imagine the hospitals have closed their doors because they are already backed up. Your grandparent is dying in the room next door because no doctor is available to treat them.
Who the hell cares if you build a ventilator and try it then? They're going to die anyways. You are doing nothing except increasing their chance of survival by acting instead of waiting.
Should you use this while hospital beds are still available? Obviously not. But any care is better than no care and being treated by a Wikipedia doctor is better than being treated by no doctor when you're already on your deathbed.
Doing something may well be worse than doing nothing.
Concrete example: you get impaled by something. Do you: (a) do absolutely nothing and leave it in, and seek help or (b) rip it out as you see in movies because doing something is better than doing nothing.
(b) will kill you and (a) will save your life.
By doing something you have no business doing, no understanding of the mechanics and consequences you may will make it worse.
If everyone in Wuhan hooked up their loved ones to leaf blowers, the death rate probably would have been massively higher.
Any other time, yes. In a time of widespread panic? Dangerous.
People are going to try to build and use these at home in an act of desperation to "do something", and end up killing their loved ones.