Personalization of logos is in a key role in communicating to visitors that your company serves similar companies to yours. If a visitor doesn't recognize a logo, the impact to conversion is much lesser than if the logos are from companies from the same industry, size, and location. Let's look at a couple of examples on how to personalize logos.
Recognition of logos on a website is important but sometimes it can also be off-putting if the geographic location1 of the logo companies is far away from the visitor's location.
For example, Stripe shows logos of Nordic companies to visitors from Nordics (makes sense, huh?). Personally I feel great seeing at least one Finnish company (Sanoma) in that line up.
Logos of US companies for US visitors. If I were to see these only, my immediate thought would be "Is Stripe suitable for EU-based businesses?"
(PS. I didn't know Booking.com was from the Netherlands, but I bet the Dutch website visitors of Stripe did).
Similarly, if the logos on a website are from an entirely different industry than what the industry of the visitor is, the potential customer has a hard time understanding whether this company understands the unique industry dynamics and problems that come with it. And it leaves room to question whether this is the best industry-specific tool to solve the problem.
For us from Markettailor, none of the logos in the Stripe example are from the SaaS industry. Therefore it's hard for new customers to comprehend whether Stripe is the best tool for SaaS businesses (the importance is highlighted when talking about SaaS and payments/subscriptions especially).
Stripe can be used for both small and large businesses alike, but for anyone not knowing the brand it's hard to know that right out of the gate. Most companies fault to thinking that the larger the companies in the logos the better2. Picking only the large customer logos alienates a lot of the smaller customers who could be bigger some day. Everyone wants to see their peers, right?
In the Stripe example, Volvo and H&M are very large Nordic companies and they don't help a small SaaS business evaluate whether it's the best product to get started with. The larger company logos definitely indicate that it's possible to scale with Stripe in the future, but it's not the best value proposition for a startup looking to save time and effort.
1) Localization is not a new topic, but it usually means changing the language of a webisite and a lot of the content to feel more native to different coutries. Creating different pages with different languages has definite SEO benefits and the logos on the website can be directly swapped for different language versions. However, the local language and country don't always match so consider combining browser language settings with the country location to create the best personalizations.
2) Having the largest logos only on the website tries to capture as large a pool of customers as possible and at the same time is mostly vanity meant for media.