1. I'm no expert, just love the versatility of these machines.
2. These machines get the most press for eating fingers.
3. Treat them with respect, they are a joy to use.
4. What HP/KW motor do you need to prevent stalling of the average sized cutter head.
In response to the points I've numbered above.
1. Agreed. They are versatile.
2. True, particularly from a historical perspective what with square head blocks and French heads, both now obsolete and long gone (hopefully) in any professional environment. I've seen people working square head blocks with the added complexity of mounting them above a dumpling and ring fence for shaping compound curves, e.g., stair handrails: a truly jaw-dropping experience to watch someone undertaking such a task, and noisy as hell to boot. I may be wrong, but I think dumpling work is now pretty much banned, or certainly highly frowned upon.
3. True.
4. ~5 - ~7.5 HP should cover it for long runs of heavy duty industrial work, including straight runs and working curves.
As to other issues or questions you raise my experience is with quite a wide range of spindle moulders from small and large Wadkins, SCMI, Felder, Hammer and a couple of other brands the names of which escape me now, with power ranging between about 3 HP to 7.5 HP, and complexity ranging from basic to sophisticated.
Nowadays, with square heads and French heads pretty much out of the equation, the main cause of safety issues are the machine's versatility. Take a table saw or planer/thicknesser for comparison. Without diminishing the ability of these machines to cause injuries if used irresponsibly or ignorantly, there's little complexity in their use compared to a spindle moulder. A table saw rips and cross-cuts wood: set the blade height/tilt, fence(s) and stops right along with the guards and cut the wood, with push sticks if appropriate. It's similar with planers and thicknessers: their only job is to flatten, square and thickness wood, again with appropriately set guarding. These machines are therefore relatively simple with limited learning to operate safely. And a mention should be made about suitable dust extraction, which applies to all machines, of course.
A spindle moulder offers the woodworker a wide range of operational possibilities, e.g., straight runs, tilted blocks, sliding tables, ring fence operations, planer head work, bearing mounted blocks, a multiplicity of moulding profiles (whether standard or custom made) jigs and fixtures, false fences and plunging through, offset fences, Shaw guards, power feeders, and so on. With a spindle moulder there are a lot of both interrelated and discrete operations and possible set-ups that a user really needs to understand and be proficient at to work in relative safety. Basically, to use a spindle moulder to its full potential an operator needs to have a lot more skill, experience and knowledge about the multiplicity of operations and setting the machine up than someone using simpler machines such as the aforementioned saws and planer/thicknessers.
Others may disagree, but the above is my take on the topic. Slainte.