Despite not being a frequent poster here (we own a 20 something year old grp thingie, with two iron lumps which means most of the time I have to be practical...).
We bought the boat in 2004 in Italy and sailed (or should I say motored) her 12 - 1300 miles home. My worst fears have always been fire onboard and as such we have always had a standing order (conveyed to all visitors) onboard... Fire extingusihers in the cabins are as a escape means only... ie enabeling escape to (relative) safety, so one can set off alarm, seek help, and gain access to more heavy duty fire fighting equipment etc., which practically cannot be located in the cabins. For this above purpose powder is perfectly adequate. If a real fire gets hold on a small boat, getting to an area where an escape is available is paramount. Saving the vessel is secondary to human life! After reaching reletive safety, one may consider to fight the fire with apprpriate means, but not to the detrement of life.
In respect to Engines and electrical systems, my problem with most systems have been a combination of price, practicality and the fact that detection, heat build-up and systems for initiating the extinguishers are somewhat disconnected. Ie you need heat buildup near an often remote extinguisher, or local detector and remote release of gas. Meaning you need a full flood system for both engine and electrical systems (latter often not covered), with a vast over supply of gas and a volume which a lot of boats cannot accommodate.
... until I found a company who facilitates a way to install a system which permitts a flexible system to pull tubing along and across the most probable heat / ignition sources, both electrical and mechanical...... and will release the extinguishing gas at the closest point of heat buildup, whilst indepentently of other power supplys notifies (set of alarm) of extinguisher activationallowing skipper to take appropriate action. Apologies for pointing you to an old MOBO thread of mys installation here...