Catch #1: Fidelity will try to upsell you.
Catch #2: Index tracking? How well do they track the index? Some index funds don't buy every holding in the exact percentages as a way to cut costs. When the expense difference is 0.03% it wouldn't take much tracking error to blow up any savings. Fidelity is using an in house index, aka not an official index, for the zero cost funds in order to save money. Since the funds are new I can't tell you how well they will track over the long term.
Catch #3: Securities lending income? Mutual funds can lend out stock they hold to short sellers and earn interest in the process. Since big index funds tend to hold securities forever short sellers love to borrow from them because they know it's unlikely the fund will want the stock back during the period the short seller needs to borrow it. As a result, many index funds make substantial revenue from lending income. Does Fidelity give this income back to the investors? Fidelity's official answer is that they give some of it back. That would be a great way to get the expenses to zero. Charge zero, but then pocket the securities lending income. Vanguard gives the lending income back to investors(their owners) as larger dividends. For really big funds like VTSAX and VTI the lending income covers a large portion of the expenses, so instead of costing 0.03% VTI is really about 0.01% after you account for the securities lending income. Some funds actually bring in so much securities lending income that they are paying you to own them. Example: VEXAX has a stated ER of 0.07%, but it's outperformed it's benchmark by a steady 0.09% for almost 20 years thanks to it's securities lending income.
Catch #4: Taxes(doesn't matter in IRA): Vanguard manages taxes remarkably. VTSAX hasn't had a cap gains distribution in almost 20 years, and VTI shouldn't ever have one since it's an ETF. Fidelity doesn't have the same track record and their index funds routinely distribute year end capital gains. Several of them are expected to distribute 2019 cap gains.
I'm sticking with the Vanguard funds. The great record of index tracking, staying tax efficient, giving back the securities lending income, and investing with a company I trust is worth 0.03% for me.