Hey,
at first - amazing mods! :)
I'm struggling to find fluid burning generator efficiency but the more I try the more strange results I get. Maybe I'm doing something wrong or I miss small but significant factor. I created 2 small, isolated setups to craft stone pipes: https://imgur.com/a/LKoGUR5
For both setups the input is 1 Fuel Oil Barrel. Left setup where I used Accumulator between generator and assembling machine was able to craft 161 stone pipes, but right setup was able to craft 2099 stone pipes. Is is intended?

there's power cables connecting to your test sites to... stuff that's off screen. So... They're not isolated as you claim.
I could assume that whatever they're connected to is effecting your results. I'll continue anyway.

The efficiency of the generator should be 100%, so 1 barrel of fuel (I think fuel is 2.3MJ) and a barrel holds 50 units of fluid. So, 50 x 2.3MJ = 115MJ total.
so if you simply connected a bunch of accumulators to a single fluid generator (and nothing else) and did your test, it should result in 115MJ of charge in the accumulators.

Accumulators would make no difference to crafting in your test, because the fluid in the generator is used as and when it is needed, however, machines have a drain, so your assembling machine and inserters will be using power even when they're not doing anything. if the items to the crafting machine were supplied at different rates, you may end up with one machine crafting more than the other due to the power burned while the equipment is idle, which means it's an inefficient method to try and gauge the potential of a power supply.

Yea, I said it wrong - assembly machine is isolated.

Right setup in Electric network info has exactly:
- Right power pole: assembling machine 1 , High capacity accumulator 3 - https://imgur.com/a/8dvJ3w4
- Top power pole: High capacity accumulator 3, Fluid burning generator - https://imgur.com/a/vQd5ofu
Left setup:
- assembling machine 1, Fluid burning generator - https://imgur.com/a/gD3zcJ1

I know about darin that is why I isolated only assembling machine to measure single entity. Assembling machine and Accumulator were empty (no power) until I put barrels with fuel, the difference in time is few seconds but that not explain 10 times more energy from single fuel barrel.

Fuel value of Fuel Oil having angel's mods is 250 kJ, barrel has 50 units so 1 barrel provides 12500 kJ and youre right about charging accumulators but in one condition - input charging speed of all accumulators is greater than max power of all Fluid burning generators.

In short - efficiency of Fluid burning generator is 100% only when it is used in 100%.

Different examples using accumulator so we don't have drain problem.
3 x High capacity accumulators, Input: 3 x 600 kW = 1.8MW = Output of Fluid burning generator 1
1 Fuel oil barrel = 12.5MJ in accumulators

2 x High capacity accumulators, 1 Slow accumulator 3, Input = 1.74MW
1 Fuel oil barrel = 12.9 MJ in accumulators

2 x High capacity accumulators, Input = 12MW
1 Fuel oil barrel = 18.7 MJ in accumulators

1 x High capacity accumulators, 1 Accumulator, Input = 900kW (50% of fluid generator max output)
10 units of Fuel oil = 5MJ in accumulators (expected 2.5MJ)
So if fluid generator can output only 50% of max power then efficiency is 200%

Using Fluid burning generator 3, max output 3.6MW
1 x High capacity accumulators, 1 Accumulator, Input = 900kW (25% on generator max output)
10 units of Fuel oil = 9.7 in accumulators

So it seems that when fluid generator is working with 50% of speed its efficiency is 200%, when only 25% of max power is generated efficiency is almost 400%.

Btw. Fluid burning boilers seems to be not affected by this but I didn't check so deeply.

Back to the assembling machines, new setup:
Fluid burning generator -> 2 High capacity accumulators 3 (2.7MW input) -> Assebling machine 1 (Max consumption 103 kW, drain 3.33kW)
Crafting: Stone pipes, 0.5 sec craft, 50% crafting speed
1 Fuel oil barrel = 120 crafted pipes + ~80% crafting bar in assembling machine
12500 kJ / 103kW = 121,36 sec (pretty close to what we expected, 1.36 sec is probably loss and that 80% of crafting bar)
I intentionally don't count drain because 12500kJ /106.33kW = 117.6 sec so we would not be able to craft 120 pipes.

Now setup with just Fluid burning generator and assembling machine 1
1 Fuel oil barrel = 2099 crafted pipes (<5% crafting bar in assembling machine)
x kJ / 103 kW = 2099 sec
x = 216197 kJ so we have 1730% fuel efficiency

And using Fluid burning generator 3 we would be close to 3460% fuel efficiency but it is 2 am here and I'm probably digging too deep into this problem so I will not check that :)

This doesn't really make a whole lot of sense to me... I mean... I'm just using a base game entity, it's basically just a steam engine, except it "Burns fluid" instead of consuming steam.

if there's an issue with it, it's an error in the game engine.

Well that's one of the options to leave it as it is. I will not try to mediate between you as a mod dev and Factorio devs because this kind of actions usually brings more problems. Anyway it was quite interesting to find why I can't balance my power generation based on angel's farming and fluid generators:)