Is it intentional that there is no fluid burning boiler 5 but fluid burning boiler 4 is produces the same steam and uses the same ingredients as Boiler 5. same for boiler 4 and fluid burning boiler 3...
The real issue is that you won't have a fluid burning boiler in the early game and none for 165°C hot steam, which is the most efficient one.

Yes, it is intended. But they do all have the same efficiency.

But they do all have the same efficiency.

If you need just steam (no matter the temperature) not at all.

Yes, it is intended.

Can you explain why?

This is a Power mod. Temperature always matters for power.

If you are using Angel's mods and need steam for petrochem, using Angel's Electric Boiler is more efficient.

At the start, burning items is your only choice. As you progress further into the game, you are given additional options: fluid fuels, solar, heat sources, nuclear.

If you are using Angel's mods and need steam for petrochem, using Angel's Electric Boiler is more efficient.

But carbon is one of the first things you need, while Angel's Electric Boiler comes at a much later stage. When you are there you can just build additional boilers. Slapping in 3 Steam Engines 2 and 5 Electric Boilers instead of building just another boiler making up for the inefficiencies is just not really worth the effort unless you really care about pollution. In which case you have to go for solar anyway.
Btw. the early game is also the only stage when it makes sense tu burn liquids since you get recipes in the late game to convert fluids to solids with higher fuel values. So in the early game you do not have the option while in the late game it is a useless option. (At least if you are playing without K2 which brings its own (better) power options.)