The last thing that pushed me towards P&M was the lower stack temps = more efficiency.
This is an important detail. My/our goal is efficiency, work smarter not harder, I mean all of the manufacturers themes are based in someway on higher efficiency no matter what wood burning device it is. Some are very temper mental and we only see or read a small fraction of them. While others have little to no issues, but are we paying attention to the details of why.
There is a window from the time the fire is lit and when it reaches its most efficient burn, some are longer than others, smoke is not the tell tale sign always either. So to get the most out of it, longer burn cycles and not short cycling. Some never reach their prime because of short cycling. Some say they have to be sized right or to the Tee. But even when we size boilers , it's only sized for the coldest day of your region. Well that's only 1% of your heating season. So the rest of the time it's oversized, and oversized units short cycle. Modulating gas valves, variable speed fans are some things to adjust output to match the load. Even then the best indoor gas boilers can be oversized and why buffer or storage tanks are spec'd for the job.
Buffer tanks capture all the excess heat for later use. Anywhere from 30 gallons up to 250 gallons. Buffers also provide a good load on the boiler with boiler protection. After 300 gallons it becomes storage. This gives you maximum efficiency and control of heat usage, with little wasted to standby losses. (Anywhere from a tenth of a degree up to .8 degree per hour). Much better than storing all the water outside, where OWB's loose 8%-20% standby losses, Plus underground pipe losses. Indoor gassers don't have this problem of outside exposure, it's just limited to the chimney losses only. If you've been reading on hearth you'll see many examples of 300-500 stack temps during full burn,(indoor boilers reach max burn much faster as well) meaning they are very thermally efficient and temps under 200 in standby. Indoor units have much less volume which equals less standby losses as well. Even the outdoor gassers have less than their conventional counter parts.
Of all the indoor boilers I've seen or all the pictures on hearth, I've never seen a dirty boiler room or house. It's only as dirty as you keep it, If it was dirty so was the rest of the place. Insects prefer live wood, A good cut split and stacked seasoned source for more than a year doesn't harbor insects much unless something is rotting. There are many that store a whole seasons worth of wood inside with no issues, but Every area is different and You should always pay attention. Some use the boiler heat or make drying zones to further dry the wood. You can read for yourself about indoor boiler owners and whether they have any issues with their gassers, there are plenty of examples. Always best to get the Info straight from multiple sources instead of cherry picking one example of an owner that hadn't cleaned his boiler in 3 years.