And how many biofuel generators can a single reactor fuel? A single reactor can fuel two fully heated boilers so you have to factor in that. Also, boilers can burn more than just biofuel from the reactors which gives them an advantage in terms of fuel diversity. You don't even have to use the biofuel to heat the thing up, you can use oil or normal fuel to heat it up and help power your productions facilities that will produce the resources to make your biofuel.
And then, of course, comes the cost of the reactor if you're using Gregtech -_- Each reactor requires a highly advanced machine hull which is made from titanium, which requires an industrial blast furnace to smelt and an industrial centrifuge getting the dusts out of bauxite. If you're not using the GT recipes this, of course, isn't that big of a deal, but still.
I will agree with you on the points of space efficiency, I'm just saying it doesn't beat boilers in every aspect, especially when you're looking at the long term. Now if only there was a multiblock MJ producer that used steam like the Steam Turbine for EU, but didn't have the upkeep. That would help in terms of both space and possibly lag from all the engines running.