I agree with litterally about 98% of what you have said. But... I can't remeber the user that posted the pic in this thread earlier of his cabinet but cabinets heated with flexx watt when executed correctly are pretty efficient..wow, one of the first big debates i want a participate in. i feel left out..
OP, i believe heat mats can be used if you have a single large tarantula and dont plan on getting more, which isnt very likely lets be honest. use a stat, timer, and make sure its not even touching the tank, keep it like 6" off. they are meant for reptiles who like hotspots at least in the low 90s. not for tarantulas who are comfortable at 70. its a lot of work to ensure your spiders are comfortable, a space heater is the better alternative in almost every way.
About 10 years ago I lived in an apartment in buffalo NY that would not stay above 60 on the really cold days of winter even with the heat cranked to 80.. The windows barely slowed the cold wind and I swear the walls were insulated with pizza boxes.. I could of spent say 80-100 bucks on a couple of space heaters and an extra $100+ a month on my electric bill or I could spend $60 on MDF and build a heated cabinet.. I heated it with flexx watt and wired in 2 small computer fans( one input and one exhaust) wired to timers set to 30 mins to circulate the air and minimize hot zones.. I think the whole thing cost me about 80 bucks.. I already had the flexx watt thermostats and timer.. The fans were like 8 bucks. Lol
So in the long run to heat my collection of about 30 Ts I spent maybe 10 dollars a month after the initial cost of hardware.
With all of that said set ups like that are not needed in most situations and the best way to heat Ts is by elevating the room temp with a space heater.