While most of the retail sector has struggled over the past few years, Home Depot Inc (NYSE:HD) stock has gone the other way. Home Depot stock is up 30% in the last year, 80% in the last three years and 160% over the past five years. One key reason for the optimism toward HD stock is that its model should be more resistant to e-commerce competition — that is, Amazon.com, Inc. (NASDAQ:AMZN) — than other retailers.

That’s certainly been the case so far. Comparable-store sales in the U.S. rose 6.2% in fiscal 2016 (ending January) and another 6.3% in the first half of FY17. Home Depot comps continue to outpace those of rival Lowe’s Companies, Inc. (NYSE:LOW), which is one reason HD stock continues to outperform LOW.

One big reason for the share gains against Lowe’s, and the growth in a challenging time for brick-and-mortar retail, is Home Depot’s success with professional customers. As Nicole Sinclair pointed out at Yahoo! Finance this summer, Home Depot gets roughly 40% of its sales from pro customers, against ~30% for Lowe’s. And that category has driven recent growth in particular, with HD management disclosing on both the Q1 and Q2 conference calls that the same-store sales growth rate for professional customers was double that of DIY buyers.

For now, that’s good news. But for two different reasons, it’s worth wondering whether in the long term, the increasing reliance on professional customers might actually turn out to be a concern.

Cyclical Concerns for HD Stock

The primary concern surrounding the increasing reliance on professional customers is the potential cyclical impact should the current, reasonably strong housing cycle reverse. Home Depot comps are rising, and some of those gains are coming from taking market share. But the company also is benefiting from increased demand both in terms of renovation and new home construction. And that direct demand is going to be most notable in the professional category, which has larger spend and is more economically sensitive.

That alone doesn’t suggest that HD stock is overvalued at the moment. But from a long-term standpoint, it is a potential risk. Part of Home Depot’s current outperformance appears to be coming from the pro category. And those are precisely the customers that will be the first to lose work if macro conditions turn. For now, with broad markets at an all-time high and the U.S. economy reasonably strong, that doesn’t seem like a near-term risk. But it does leave Home Depot potentially more exposed than Lowe’s when the cycle does turn.

Amazon, MRO and HD Stock

There’s a secular concern to Home Depot’s strategy as well. HD has moved aggressively into the B2B (business to business) and MRO (maintenance, repair, and operations) distribution spaces with both its emphasis on professional sales and its acquisition of distributor Interline Brands in 2015. But in that space, Amazon may be a bigger problem than in the legacy Home Depot stores.

After all, Home Depot is having tremendous success in “omnichannel” retailing, as James Brumley pointed out earlier this month. The need for DIY customers, in particular, to view objects in person and/or ask for assistance helps limit online competition. So does the low dollar value of many home improvement items, which can make shipping costs prohibitive.

But, in B2B, it’s a different story. And it’s worth noting that the distribution space, in particular, is one where investors are showing a lot of caution. The rise of Amazon Business has sent shares of major distributors like W W Grainger Inc (NYSE:GWW) and Fastenal Company (NASDAQ:FAST) tumbling. GWW shares have recovered after a strong earnings report, but they still fell nearly 40% just between February and August. FAST stock still trades below early-2012 levels.

Mid- to long-term, the category where HD is having the most strength right now may turn out to be its most competitive. Pro customers generally know what they want and for how much. Home Depot’s delivery efforts and incentive programs mean it will likely keep most of those customers. But the Amazon threat HD bulls dismiss in the consumer business may be larger than some realize on the professional side.

Bottom Line on Home Depot Stock

To be sure, those risks don’t suggest a short of HD stock, or anything close. This remains a dominant franchise, with impressive earnings growth. The dividend yields over 2%, and Home Depot has raised its targeted payout ratio to 55% from 50%, implying larger increases going forward. And a ~20x forward EPS multiple isn’t particularly aggressive, especially relative to recent growth.

But an investor still needs to trust the cycle to buy HD at current levels, and trust that the company can manage the Amazon threat in B2B. On both fronts, I’m optimistic, but cautiously so. And I see enough risk to keep HD stock from being that compelling just off all-time highs.

As of this writing, Vince Martin has no positions in any securities mentioned.