Performance Max is Google’s most polarising campaign type. Half the PPC community thinks it’s a breakthrough in automated advertising. The other half thinks it’s a black box designed to spend your money on junk inventory while making the numbers look good.
I manage PMax campaigns for eCommerce clients every day at RTN Digital. My position: PMax can work well for eCommerce, but only if you understand what it’s doing, control what you can control, and measure it properly. Left to its defaults with no oversight, it will spend your budget in ways you probably wouldn’t choose.
The PMax transparency problem
Performance Max rolls Search, Shopping, Display, YouTube, Gmail, and Discover into one campaign. When it launched, Google gave advertisers almost no visibility into which channels were spending what. You’d see a total spend number and a total conversion number, but the breakdown of where those results actually came from was hidden.
This was the original and legitimate criticism. If you can’t see that 40% of your budget is going to Display placements on mobile gaming apps, you can’t make an informed decision about whether that spend is driving real value.
Google has improved reporting significantly since launch. In early 2026, they introduced channel-level performance timelines that show spend and conversion breakdowns by network. They expanded negative keyword capacity to 10,000 per campaign. They added placement-level reporting that lets you see exactly which websites and apps your ads appeared on.
These improvements matter. But they only help advertisers who actually check the reports and act on what they find. And in my experience auditing accounts, most don’t.
Where PMax budgets typically go for eCommerce
For an eCommerce brand with a product feed connected, PMax will typically allocate the majority of spend to Shopping placements. This is usually a good thing because Shopping is where high-intent product searches live. If someone searches “buy waterproof hiking boots size 10,” Shopping is the right place to show them your product.
The problem areas are the other channels. Display and YouTube placements tend to receive a portion of spend by default, and the quality of those placements varies enormously. I’ve seen PMax spending eCommerce budgets on Display ads shown in children’s game apps, on YouTube pre-roll for videos completely unrelated to the product category, and on Gmail promotions tab placements where the open rate is negligible.
None of that spend is necessarily wasted. Display and video can drive awareness that eventually converts. But if you’re running a £3,000/month PMax campaign and £800 of it is going to Display placements that generate zero trackable revenue, you need to know about it and decide whether that tradeoff is worth it.
The brand cannibalisation issue
This is the one that burns eCommerce advertisers most often, and many don’t realise it’s happening.
PMax will bid on your brand name unless you explicitly exclude it. When someone searches your brand name directly, they already know you. They’re navigating to your site. They would almost certainly have found you through organic results. But PMax bids on that query, shows your ad, they click it, and PMax claims the conversion.
Your PMax ROAS looks fantastic. But a meaningful chunk of those conversions aren’t incremental. They’re existing customers and brand-aware shoppers who would have bought anyway. You’re paying for traffic you’d have got for free.
The fix is straightforward: enable brand exclusions in your PMax campaign settings. Google lets you exclude your own brand and any other brands you don’t want to bid on. If you also run a dedicated brand Search campaign (which you should), this prevents PMax from cannibalising it and gives you a clearer picture of what each campaign is truly contributing.
One of our bushcraft and outdoor clients saw their true non-brand PMax performance come into focus after we implemented brand exclusions. The headline ROAS figure dropped, which initially looked concerning. But the revenue stayed the same because that branded traffic simply shifted to organic and the brand Search campaign. What we gained was honest data about whether PMax was actually finding new customers. It was, and the non-brand ROAS was still strong at over 10/1.
How to run PMax properly for eCommerce
Based on what we’ve learned managing PMax across our eCommerce client base, here’s the approach that consistently delivers:
- Start with your feed, not your campaign. PMax Shopping performance is directly tied to feed quality. Optimise your product titles, descriptions, images, and custom labels before you even create the campaign. Weak feed data means PMax has poor raw material to work with, and it will underperform regardless of your budget or bidding strategy. We run feed optimisation as a prerequisite for every PMax campaign we launch.
- Enable brand exclusions from day one. Don’t wait until you suspect cannibalisation. Just turn them on. There’s no downside if you’re running a separate brand campaign.
- Set up asset groups around product categories. Don’t dump your entire catalogue into a single asset group. Segment by product category or margin tier so PMax can match the right creative to the right audience. Custom labels in your feed make this easy.
- Review placements weekly. Check where PMax is placing your Display and Video ads. Exclude anything that clearly isn’t relevant to your products or audience. Yes, this is manual work. Yes, it’s worth doing.
- Run a holdback test if budget allows. The gold standard for measuring PMax effectiveness is a geo-based holdback test. Run PMax in some regions and not others. Compare total revenue (not just ad-attributed revenue) across both groups. This tells you the true incremental impact, not the inflated version PMax reports.
- Don’t rely on PMax alone. We typically run PMax alongside a standard Shopping campaign and a separate brand Search campaign. The Shopping campaign protects your bestselling products with more granular bidding control. The brand campaign captures navigational queries without paying PMax prices. PMax handles the broader prospecting.
The verdict
PMax is a powerful tool for eCommerce when managed actively. The automation genuinely does find pockets of demand that a manual campaign structure would miss. But automation without oversight is just a machine spending your money according to Google’s priorities, which aren’t always identical to yours.
Google’s incentive is to maximise spend across their entire ad network. Your incentive is to maximise profit. Those goals overlap, but they’re not the same. The gap between them is where money gets wasted and where a good PPC manager earns their fee.
If you’re running PMax and haven’t checked your channel breakdowns, placement reports, or brand exclusion settings recently, do it today. And if you want us to do it for you, our Free Ads Analysis will tell you exactly where your PMax budget is going and whether it’s working.