This your doubt is similar to the doubt between creating a site from scratch or using a WordPress, I think the advantages and disadvantages are similar.
In my experience using a BaaS (Firebase) are:
Advantages:
You do not have to take the time / money to create a backend
Hosting costs for your back-end in the cloud are often
more expensive than the cost of BaaS
Your backend is updated to
better security and optimization without the need for you to do
nothing
You have good integration between services
Other services such as bug testing are also offered by these systems
If the big players in the market are creating this type of service it is because it adds value and there is market
Disadvantage:
You only depend on the functionality that BaaS provides you
You must trust your data to the service provider
You are relying on the service security system
Some functions may be limited to your application
You may be kicked out of the service for some reason and lose access
You depend on this company, if it closes you will have to migrate to
another service provider
It may seem like there are a lot of disadvantages, but I believe that only a very specific application does not fit BaaS, I use Firebase and it supplies all my needs, plus a very generous free version.
