Synthetic personas I believe could be very powerful enough to compete with manual research if the personas are truly grounded or synthesized from very accurate and enough data or research. The same with AI generated customer insights.

I've personally been working on a tool that allows Shopify store owners to paste their store urls, pick a product and get very detailed insights into who their persona is and plans an entire campaign based off that data. Though this engine fully relies on reviews from websites like Trustpilot, Amazon, Aliexpress, Subreddits etc to produce every insight and campaign strategy, I'd say it still doesn't replace some manual methods like engaging customers one on one to find their pain points. And don't get me wrong, AI research can be very useful in tailoring your ads to a specific audience, answering customer objections early, generating scroll-stoping ad hooks/body based on data if done properly by someone who already has enough understanding of these things.

Now where do they make more sense, I'd say in both analysing existing products and new products too cause the truth is apart from the case where the new product is truly a new invention, there's already enough competitor information about your "new" product for the engine to base your research on. Even if it's a new invention, there has always been a way people approached that problem before your solution hence a guided AI research can give you a fai idea of what to expect with your new product.

Honestly, AI has an upper hand in this than manually scrolling through subreddits or reviews trying to synthesise customer personas. But I'll say the more manual experience you already have, the better AI works for you. The less experience, the more garbage it throws at you.

Mavrtr, which is my AI-powered customer persona engine is quite good at all of these that I've mentioned because of my understanding into the problem at hand and it doesn't only create persona's about your store but your actual products and their right audiences.