Weebly SEO is like SEO on any other platform. The PHP generates HTML, which is all Googlebot and any crawler for that matter can see. Therefore it makes no difference.
The only problem with a lot of content management systems (like Weebly) is that they lack flexibility. The reason Wordpress is so big is that it's great for beginners, but it's also very flexible - so you get the best of both worlds.
That being said, if you aren't techy, so long as your website is high quality (content, images, metas, title) your website should still compete for less competitive niches.