I can see both sides of this, tbh.
I fully understand that creating from scratch a system to manage this will take time, a lot of work and that there will be cock-ups That's true of any new system thrown together like this. I'm sympathetic to that and I don't think there will be a deliberate intention not to pay.
BUT, what's making businesses nervous is that they have actually dealt with HMRC before - and what they know and the public don't is that HMRC is notoriously, famously, completely and utterly incapable of managing its existing tasks to any sort of standard, much less coping with something this new and hurried. Its existing IT is shit, isn't integrated across departments, its records keeping is shockingly poor and staff actioning tasks and communicating adequately and on time scales that would be acceptable in any other field just doesn't happen.
Bluntly, they've been hiding for years behind 'we're HMRC - everything is YOUR fault, you pay your accountant to fix OUR error', and now they're going to struggle, because they don't have the beginnings of the set up or the capacity to do what the government has promised they will. RTI (which will be the basis for eligibility) is glitched as anything and resolutions to Revenue caused (not business caused) errors literally take years, (and I do mean that exactly as I say it - actual years).
Anyone who thinks the Revenue will manage this seamlessly and with good style, has simply never dealt with them. Businesses know that and they're nervous.
Coupled that with the fact that this is supposed to be a 'refund' of wages already paid and 'nervous' will become, for a lot, 'untenable'. There's a rhetoric in the country of 'fat-cat' directors raking it in and businesses swimming in cash and just saving it up to pay dividends instead of more wages as a choice and , while that may be true in some cases, it certainly isn't for a majority. Payroll is often a companies' biggest bill, and a lot of none-trading businesses will already have used up their cash-at-hand on the end of March payroll and the trade debts which fell due then. They just won't have the funds to make payment without this money coming in first.
There are a thousand things that could have been done to ease this - the biggest and simplest of which would have been offsetting falling due PAYE as a start. If they'd done this in early or mid-March, then this would have relieved businesses of the need to transfer INTO HMRC by the 22nd of March and of April. The PAYE due bill is around 1/3 of net pay to staff on average, so 'standing down' those two payments would have given a lot of business an immediate boost of 2/3 of the End April payroll run - not far at all from the promised 80%.
And, to make it worse, the loan scheme that should have plugged this shortfall isn't working. Banks aren't set up to cope with the demand and a lot haven't stuck to the spirit of it at all and have been coming up with crazy schemes like 22% interest loans secured against directors' homes and not offering the government backed loan at all.