Back in the day when I worked Big 4 FDD, the entire tax diligence group was made up of tax lawyers for reference. Tax diligence typically includes entity structure analysis for tax efficiencies. And then if you're looking at a stock deal the historical accuracy of the filings becomes a lot more important. A good tax diligence team should also be able to point out any tax saving opportunities or potential future expenses the prior owner ignored paying/filing. Sales tax can be a big one here.

It often makes sense to use the same firm you are using for financial diligence (to the extent that firm has the capabilities to assist). It is also important to differentiate tax diligence (federal and SALT diligence), from tax structuring as part of the deal. Often the tax diligence team can perform both services, but you would want to make sure that you are covered on the tax diligence, and not just by a tax attorney that typically works on the structuring aspects.