I would think the biggest concern with "Serverless" would be vendor lock in. If a large portion of your SaaS product is "serverless" it's going to be very difficult to move when company A raises their prices, or company B comes in with a much more compelling product or price point.

Admittedly I don't know enough about the details to know how big of a deal this could be, or whether there is work toward a universal "serverless" standard. But I don't want Amazon/Google/Whoever suddenly raising their rates 2 years from now causing my startup to go under because the margins are so tight. And for those that say this will never happen, look at what Google just did with their Maps pricing.