Pretty much that. Boilers have to be fueled constantly and the biggest and baddest out there produces 144mj/t worth of steam. At max temperature it is 45% more fuel efficient than the other engines and can run off of fuel, biofuel, methane, and creosote oil. A single bucket of fuel will run it for 5 minutes and 3 seconds, so a single fuel bucket can produce roughly 872,640 mj, give or take some due to rounding. This is compared to a single combustion engine producing 600,000 mj per bucket at 6mj/t.
Of course, you can tweak the boiler size as well - smaller boilers don't take as long to heat up and use less fuel, but produce less steam. End game they're very fuel efficient and due to being able to run on biofuel and methane (gregtech) can be easily automated. The problem is when people can fairly easily build a massive amount of magmatic engines and pump the nether dry it doesn't seem as appealing to some people, even though it requires no player input once all of the automated processes are put in place.