A few have it right. Per diem deduction will not be allowed for personal filers starting 2018 tax year, in lieu of the greater standard deductions. However, a business can still deduct it. So many are substituting part of the wage with a per diem payment that is not taxable income, since it is an expense the employer is reimbursing.
From an employee's perspective, it's like getting your per diem deduction back on each check versus waiting till the end of the year. That is, due to the lower tax withholding resulting from having less taxable income due to part of it being called a reimbursement. Versus just taking the bigger standard deduction and living without the extra deduction. Now you get both anyway.
On the employer's side, the reduction on withholding amounts is equal. Additionally worker's comp premiums are reduced, so employers will benefit from that. Downside for the employee is one WC benefit is a partial wage payment for downtime due to an on the job injury. Not sure of the exact percentage, but of course a percentage of a smaller income number will yield a lower benefit payment.
The lower reported income could bump you into a lower tax bracket if conditions are right, so it might not be all bad in some cases.
Also notable the employer can deduct 80% of the payment, so somewhere around $24 on $100. Since it takes the place of withholding contributions and WC premiums, it's not exactly a windfall.
IMO the employee retaining that "deduction" money is more favorable than not. What I do see is an unfortunate opportunity for dishonest employers to game this thing against employees. The CPM reductions and "extra charges" being discussed for example. I'd be suspicious as hell about that. As an employer, I'm able to do the little extra accounting to just do the max $63/$47.25 and keep it simple.
In my case, I have three employees including me. Two of us drive (the other driver is my son) and qualify for per diem payments. I offered the option to do it or not. I walked him through the scenario above and he's elected to do it. I am also a w2 employee and am doing it.
Until I see some evidence to the contrary, I see a minor benefit to the business. My payroll settlement is about the same as it was before. I think the employee benefit is greater, since we still get the value of the old per diem deduction AND the larger standard deduction at year end too.