ADP has run payroll longer than most of its competitors have existed, and RUN is the small business product. You buy it for the compliance machine behind it, not for the interface.
What it does well
When a tax notice arrives, ADP handles it. That is the thing customers actually pay for and the reviews bear it out: the resolution rate on agency notices is the highest in this category. For a business that has been burned by a penalty, it is a rational reason to pay more.
Coverage is broad. Multi state, complex pay rules, garnishments, unions, certified payroll for construction: RUN handles cases that trip up newer products.
Tax and filing
US filings are comprehensive and automatic: Form 941, 940, W-2, 1099-NEC and state and local filings including the local jurisdictions that Ohio and Pennsylvania businesses know about.
ADP does run Canadian payroll, but it is a different product on a different contract, not RUN. T4s, ROEs, CPP and EI are handled there. A business with staff on both sides of the border can stay with one vendor, which is unusual in this category, but expect two agreements.
Where it frustrates people
Price is quoted, not published, and it is negotiable, which means customers on identical headcount pay materially different amounts. Add on fees for year end and for W-2 delivery are a recurring complaint. The interface is dated. Cancelling is harder than signing up, and the reviews are blunt about that.