It's an issue because it does two things:

1. Favors certain types of businesses that can more readily arbitrage their domicile (e.g. Apple, Google).

2. Creates an incentive for companies to pay accountants and lawyers a large amount of money to find ways to engage in #1. All of this money is, in economic terms, deadweight loss.

If you look at the effective tax rates companies were paying before this change, they were more or less harmonized with the rest of the world[1]. What was not harmonized was the statutory rate. What this meant in practice is that big companies with the resources to exploit the loophole paid the low rates, and companies without those resources paid the high rates. And the resources that were expended by the companies to accomplish this were 100% unproductive. Harmonizing the statutory rate makes things fair, and eliminates the need to waste money on those types of accountants.

[1] https://www.cbo.gov/publication/52419

"2. Creates an incentive for companies to pay accountants and lawyers a large amount of money to find ways to engage in #1. All of this money is, in economic terms, deadweight loss."

I would argue that is not an issue because that happens anyway. There is not, nor will there ever be a tax point where these huge companies don't attempt to get out of paying taxes.

Not true. Complexity in the tax code incentivizes gaming expenditures. Companies invest in things that generate return. When the gain from investing in accountants exceeds the cost of those accountants and their sheltering strategies, they invest in it. A good tax code makes the gain to be had from loopholes smaller than the cost of implementing those loopholes. This corporate tax rate harmonization was one step among many more that ought to be taken along the path to doing that.