I think there are fundamental different in technology perspectives around these platforms.
But, we can only guess it, unless those founders are showing their building blocks.
Flutterflow is more like automation tool for developer, so coding skill is required, but not as intense as coding from scratch, without coding skill it would be difficult to achieve final product.
But for runtime, this will be the quickest and future proof (or present proof) to engine breakdown. Just need to get our hands dirty with source code.
Bubble and Adalo also has a difference in my assumption, Adalo have not managed to separate the editing and runtime algorithm, so sending request and receiving data both for editing and runtime are using the same method, sending json data.
There are chains that data has go from postgress (real db) to list on screen, it could be 10 gateways or less or more, while each gateway will package the data with transformation and bundling, so any data we request, it will go through these gateways back and forth, so that is why a member here identify all API data will come from Adalo server instead of direct from the source, because it needs to repackaged using those gateways.
What I think that Bubble has manage to do that separation, but maybe not fully, that is why they can give more or limit hardware related resources, so the client is no longer just another engine that need to open repackaged data, but it becomes the code itself, which is at par to developing front end client from code.
In simplified terms, no code client is still an engine, engine is slow, it needs to be converted to just client without engine, to do that no code server must change to serve the need for no code client and just client (translated from no code client).
It is mouthful indeed, so had better expecting explanations from official sources.
But in practice, I have seen some progress, many to many relationship updates are quicker, count handling is faster too, but it could be the peak time of those gateways, I may use them during low peak period.
My suggestion is if you have time and effort, embrace all, they are all useful and can be used at particular work when regarded as suitable.