Hello tigboy, since I've designed and built my own wood/coal boiler I'll offer you my opinions and experiences.
First, when you put burning wood into the equation, you limit your options for an efficient waterjacket/heat exchanger design. The reason is that the waterjacket/ heat exchanger design for a wood stove must be simple with easily accessable surfaces, with cleanout doors and access panels. Wood smoke condenses on surfaces cooler than 250-300*, leaving hard, crusty creosote behind. You must be able to scrape the creosote off the inside of the boiler to keep the boiler's efficiency up. The creosote insulates the waterjacket from the heat of the fire.
With a coal only boiler, the by-products of coal are heat and fly ash. Fly ash will fall off or can be vacuumed off the waterjacket/heat exchanger surfaces in a coal boiler. In an EFM or Gentleman Janitor, Keystoker, Harman and other coal boilers, there are many surfaces, mostly vertical that the hot coal-heated air pass over and between exchanging heat into the waterjacket. These many surfaces are inaccesable to scrapers that would be needed to remove creosote, but these surfaces can be cleaned with just a brush, or stiff wire and a vacuum. The fly ash just falls off. The efficiency stays high.
So for wood burning, the waterjacket/heat exchanger design must allow scraping and removal of the crusty creosote. I made my 'Big Bertha' boiler with all flat internal surfaces, including tubes above the fire, all accessable with a small garden hoe and scraper, all sweepable and accessable with a shop vacuum. Extra cleanout doors, and the entire boiler made of Stainless Steel to offset the effects of the corrosion from creosote deposits.
I have a lot more surface area inside my boiler than usually found in wood burners, but much less compared to the average coal boiler this size. So my 'Big Bertha' is a very good wood burner, but not so good at recovering heat from coal. I also made it have a huge water capcity to absorb temperature and fire overshoots.
The control of a hand feed boiler is based on water temp and an aquastat, the aquastat controls a combustion blower and often a moveable flapper over the air intake for intake air control. But, once you have a large wood or coal fire burning, you cannot very easily slow it down to an idle. Shutting off the air will still take many minutes to reduce the heat output of the fire. So overshooting the target water temp is planned on and common.
A typical scenario that results in an overshoot would be the morning ritual: people wake up, thermostats are moved to warmer temps, either automaticly or by hand, and people start to take showers. All these put a large BTU load on the boiler, so the combustion fan runs, the fire gets really going, creating lots of heat to heat the house and domestic water. But suddenly, everyone gets in their cars and heads off to work and school. The thermostats are turned down, no more hot water is being used. The big hot fire is roaring along, putting 140,000BTU per hour into the water, but the water is no longer circulating, the water soon gets too hot, and the fire is STILL not back at an idle..
So a boiler needs a 'heat dump' circuit, one that can be automaticly turned on to 'dump' heat when the boiler threatens to overheat and boil. Or a boiler needs to be set to run at say 150*, and have large enough water capacity to absorb the heat and not boil.
The introduction of oil and gas burners that could instantly remove all heat from a boiler made the above scenario no longer a problem.
So a big hand feed boiler has a problem it has a big fire to make lots of heat, but the instalation must be done so that if it overshoots the water temp significantly it has a way to dump the excess heat. Or it must be able to store it in excess water. The problem with excess water is that it takes lots of time to heat this water up to a funcional temperature once it drops below target temp.
Stoker wood boilers can almost instantly reduce the heat output by stopping the feed of fresh coal, and stopping the combustion blower, the fire idles down very fast, almost eliminating overshoots. But you can't burn wood in a stoker boiler.
My fingers are getting tired, I hope I've answered some of your questions, and not created too many more.