I worked at a software house as a creative dir. for a bit and created animations for Lottie as well.
I'm not very familliar with the developer/programming side of things but one common feedback I got from the developers was that Lottie can cause some delays. It's not about the weight of the files (JSON files still beat the crap out of any gif or mp4) but more about accessing the Java Script Lottie library.
This is not really an issue if you want to just have a vector animation play on the website. The delays start to be noticeable if you want to add a few interactions (which is the best part of using Lottie). For example - with every 100px you scroll down the site, the animation changes by accessing/ jumping to a different part of the animation. With these kind of things, apparently it's not easy to get a fast and fluid effect .
From a creative standpoint - I think Lottie is great. Being able to create e.g. a vector animation of a character which "weighs" 30KB which would weigh 2MB as a gif. Of course Lottie doesn't support many AE effects so you have to be careful what you use in AE, depending what platform the animation will be used on (mostly rely on simple shape and path animations). Effects supported by Lottie are updated from time to time and you can check them on their website. https://airbnb.io/lottie/#/supported-features