I have some BR-IM45s, and some BR-IM70s (which have large cooling fins), on a couple of my bikes, although they're only used for relatively lightly-loaded commuting (typically only around 20m per day, load of around 15kgs, rolling along at 15mph). They're actually pretty easy to strip down and service, though this simply consists of disassembly, degrease, regrease and re-assembly. IME, under these conditions, the brakes need a strip down and relube about every 6-9 months to maintain the best performance (decent modulation with good braking power).
On the BR-IM70 front, for example (please see photos), the shoes brake against the inside of a 'drum' which is part of the cooling disc, so I imagine the heat in the disc itself can escape fairly well, but the heat in the shoes can only really travel through the grease and on into the disc, which I doubt is a very effective method of cooling the shoes, especially as they, like the 'drum', are made of metal.
If the brakes (well, mine, at least) aren't relubed regularly enough, they tend to lose braking power, unless the lever is pulled quite hard, at which point they 'grab' and brake sharply. This wears the metal shoes inside the brake quite rapidly, and if it goes on long enough, permanently reduces the braking power. For example, the 45s I've got were on a used bike I bought, and despite a strip-down and regrease, have never braked particularly well - they just don't have the stopping power any more.
BTW, just squirting more grease into the hub, as Shimano recommend, is not a very good way to keep the brakes running; all it does is mix old grease with new, with mediocre results. It's much better to degrease the parts completely, and then regrease.
In terms of fast descents, I've obviously never had to push mine on a commute, but I tend to slightly alternate the braking power between front and rear brakes on descents, and I've not had any problems with my roller brakes cooking grease. With a heavier bike on a longer descent, I'd feel confident on a mild gradient, but I'm not sure I'd bet my life on them going down a steep slope, unless I was keeping my speed down most of the way.
If I was back doing some relaxed touring with the ~15kgs I used to ride with, and not regularly doing a lot of mountainous riding, I'd personally be happy using roller brakes, but then I tended not to ride for more than 2-4 weeks at a time anyway.