I obviously don’t know everything about your situation, but typically even if you go with a big four firm for your tax return (with foreign income) the IRS likes to dig into everything. So even if the tax return is 100% correct sometimes the firms know the laws better than some IRS agents, and ultimately this means your tax returns get tied up in red tape (notices and audits).
Source: Inpat/Expat tax accountant.
Also I’m not going to say that no other firm can handle foreign affairs, but if you have significant foreign travel or activity it makes sense to have a large firm do your return.
Other source: Have amended a number of fucked up returns from smaller firms.