One concern with a job guarantee being proposed in isolation is that opponents clearly attempt to paint the policy as workfare and, possibly worse, right-wing advocates or implementers of the policy would attempt to introduce it in that pernicious form rather than in the way that the job-guarantee advocates clearly intend, which is as a job with minimum wage and basic conditions attached. There is no doubt that the job guarantee, as intended by modern monetary theorists, is very different from workfare.
At the risk of sounding like a broken record, I think that introducing a basic income in conjunction with a job guarantee to form a 'job and income guarantee' would be the surest protection against the job-guarantee concept being twisted into workfare. The existence of a basic-income option would strengthen the position of the unemployed, because if the job guarantee was considered unsatisfactory in its implementation, there would be a viable alternative.
Likewise, by ensuring a better job-guarantee program, the position of workers in the broader economy would also be strengthened, because the job guarantee and basic income would both provide them with better alternatives than they currently have. This would give workers a bit more bargaining power over wages and conditions.