Lost in all the early anxiety about QBI seems to be something else that will affect far more taxpayers -- the loss of deduction for equity mortgage interest. The DIYers will probably just ignore it for the most part and continue to plop the Form 1098 number into their return, unadjusted.
My best tactic so far is to compare the mortgage balance reported on Form 1098 to the assessed value for property tax purposes, which in California is actually a somewhat realistic indicator of the cost of purchase/improvement (thanks to Prop 13). Of course there are lots of exceptions to that, but it's a starting point that does not require any time spent with client. If the numbers are reasonably close, I'll probably just assume it's all acquisition debit.
And yes, I always considered this issue when it came to AMT adjustments, even before TCJA, but it was the rare client who was both subject to AMT and who also paid equity debt interest. Now it's potentially every client who itemizes with mortgage interest.