Buckle carriers and mei tais can be VERY different in the "coolness" factor due to the material. There will be mei tais that are hot because of heavier fabric and some that are cooler because of lighter weight fabric and the same is true with buckle carriers.
So it depends on which specific mei tai you were looking at.
You want a sturdy bottomweight or twill or lightweight denim for the base fabric and straps, but not another thick layer of fabric like decorator fabric if you want to keep it cool.
My Blessed Baby is a hybrid carrier that buckles at the waist but ties for the shoulder straps. It's not a heavy fabric so it's fairly cool to use in the summer months. This is one of the carriers I'll be using when it gets warm.
www.my-blessed-baby.com
The ties for a mei tai are no less safe than the buckles on a buckle carrier IMO. If you know how to tie a bow or a knot then you're safe. I personally think I can get a mei tai tied more secure than I can get a buckle adjusted with an Ergo or buckle carrier. It's alot about personal preference.
