Ahrefs vs Semrush Site Audit: The Technical SEO Truth

I spent a few weeks running the same sites through Ahrefs, Semrush, and my own crawler to figure out which one I’d pay for if I only cared about technical SEO. The big suites will tell you a site is healthy right up until traffic starts sliding. Here’s how they compare, the three gaps both of them share, and who should use what.

What Ahrefs and Semrush are really selling

Both are marketing suites first and audit tools second. You’re paying for backlink databases, keyword research, rank tracking, competitor dashboards, and a hundred tabs you’ll click once. The site audit is a feature inside a monthly subscription, and the price climbs as you add projects, users, and pages.

If you need all of that, pay for it. If your job is technical SEO, most of what you’re buying sits unused.

Ahrefs: fast, but shallow on JavaScript

Ahrefs is quick. The interface is clean, and the way it groups issues makes sense most of the time.

Where it fell apart for me was JavaScript. I tested it on a React single-page app I was consulting on, and the audit reported pages as empty or missing titles. The titles existed. They were injected after the initial load, and the crawl didn’t wait for them.

If your site is server-rendered HTML, that won’t bother you. If you run Next.js, Nuxt, or anything hydrated on the client, expect false positives you’ll then have to explain to a client who already paid for the tool. I went deeper on this in the Ahrefs site audit alternative write-up.

Semrush: integrated, but slow

Semrush’s pitch is integration. You click from a keyword report into a page audit, and everything more or less connects. If you already live inside Semrush all day, that works.

The audit itself was slower than Ahrefs in my tests and not meaningfully better at rendering modern JavaScript. It catches the basics and misses the weird stuff. It also flags plenty of “issues” that aren’t issues, which trains people to ignore warnings. That’s a bad habit to build. The Semrush site audit alternative post covers its quota and rendering limits in more detail.

Three gaps both suites share

1. JavaScript rendering that passes but isn’t fine

A client had a product catalog that loaded its real content client-side after a fetch. The initial HTML was a skeleton loader. The suite audit saw a 200, saw some text, and gave it a green light. Googlebot was indexing the skeleton.

The audit wasn’t comparing what the server sent to what the page looked like after rendering, so it never flagged the gap. Your H1 appearing only after hydration is exactly the kind of thing that quietly kills rankings while every report says the page is healthy. The JavaScript rendering issues that hide from audit tools go well beyond missing titles.

2. Server logs are invisible to them

Suite audits don’t touch your server logs, so they can’t tell you Googlebot is spending most of its visits on a faceted navigation black hole you thought you’d blocked.

I watched a client burn crawl budget on ?sort=price&color=blue&size=... URLs for weeks. The audit said no issues, and robots.txt was blocking the pattern. The logs showed Google requesting those URLs anyway, because a stale internal link in a footer widget pointed at a variation the robots rule didn’t match. You only catch that by putting your own logs next to a crawl.

3. Redirect chains reduced to a two-word summary

Suites will flag a redirect chain. What they won’t show you is the chain that includes a protocol flip, a subdomain hop, a country redirect, and a canonical pointing at a third URL.

On one multi-region ecommerce site, the audit said “2 hops, fix recommended.” The actual path was HTTP to HTTPS, non-www to www, a country detection redirect, then a 302 to a localized URL whose canonical pointed back at the original. Google was confused. The tool was not concerned.

Why this keeps happening

The suites are built around a score. A score needs inputs that are cheap to compute at scale and look good on a dashboard card: missing alt text, broken links, short meta descriptions. Rendering comparisons and redirect forensics are expensive and hard to visualize, so they get skipped.

Where a dedicated crawler wins

I built Black SEO Analyzer because I was tired of explaining why a suite said a page had no title when the title was right there in the DOM. With --spa it renders every page in headless Chrome and grades what actually loaded. Every URL’s status code, redirect target, and HTML lands in a local SQLite database, so walking a messy redirect path or joining a crawl to your own log export is a query, not a support ticket. It runs locally, has no page caps, and costs a one-time license instead of a subscription.

From my comparisons:

• JavaScript sites. The rendered crawl caught lazy-loaded content and client-side meta tags the suites missed on the same URLs.
• Dynamic routes. A rendered crawl followed links that only existed after the router mounted. The suite crawl stopped at the shell.
• Raw output. JSON, CSV, XML, and HTML, so the results go into scripts and tickets instead of screenshots.

It is not a marketing suite and isn’t trying to be. If you need backlink data, Ahrefs is good at that.

Who should use what

• Agency running 40 client campaigns across SEO, content, and ads: Semrush or Ahrefs. You need the suite.
• Programmer or technical SEO auditing a JavaScript-heavy site: a dedicated rendering crawler. It’s cheaper and more accurate for that job.
• Founder doing a one-time audit before a relaunch: don’t sign up for a year of anything.
• Already paying for a suite but seeing green reports while traffic slides: keep the suite for keywords and run a real crawl next to it. If the audit says green and traffic is falling, the audit is wrong.

Run your own comparison on a site you know well. BSA is free for 14 days, every feature, no page limit, no credit card.

-Sethers