I got this question from a person in an early stage startup: Do you consider Design to be part of the Product team, or a separate department which works in tandem with Product?
Before I show my answer, which might also apply for non-early stage companies, I would say that most early stage startups benefit from having a full-time designer, often way more than having a full-time product manager. In general my advice is that founders can cover the product part of the (early stage) business, but the design role requires more specialized skills. So having a designer and an (lead) engineer reporting to a founder is to my experience the most effective startup org chart I have seen for most pre-PMF startups.
But as you can see below, there are plenty of nuances here and it might be that the context you face requires something different. So, here is my answer to the question:
There isn't a definitive answer here and it really is context specific, so let's break it down into parts:
1. The first assumption is that a product squad is composed of engineering, product and design. In my opinion those roles should be (sort of) horizontal, meaning that inside a squad there should be no power of one area over the other. The composition and balance of the three is the key of a great product.
2. Then how do you hold people accountable if they spread the responsibility? Well, you hold the whole squad accountable - everyone pursuing the same goal. If you must go more specific you make (i) engineering responsible for code delivery and technical feasibility risk of the product; (ii) product responsible for customer and business value risk of the product; (iii) design responsible for the usability risk of the product and customer experience while using it. But again, either the product works to deliver business results or not - everyone should be accountable for that.
3. Now it comes to the core of your question: should everyone be under the same leadership? The funny thing here is that people usually split engineering and product/design and the question often comes to whether product and design should be in the same team - I do not hear the question that often whether engineering and product should be in the same team. So, no matter your decision here, if engineering has a different leadership you will need to create processes to make different areas work together under the same squad.
4. If you separate the leadership you have the pro of a specialized leader that will probably develop the team better and bring more domain specific knowledge to the table. If you keep everyone under the same leadership you have the advantage of centralizing decisions and making everyone part of the same team by design.
I'm getting long in my answer here to make you think in your own context. What do you want as a company regarding the product and design teams? The answer is more nuanced than most people think. But early stage startups are a bit different in my opinion. You have no time, fast go to market is key. In general I would recommend for companies at this stage to centralize decisions in the founders or as few as possible leaders - you can always change that later. But again, it might be that your scenario is different - that is why I don't want to be so prescriptive.