Upsides:
you can load "short rolls" if you know you're wanting to get a particular shot, that you'll only need half a dozen frames for, and you need to process it for a "later that evening" deadline - so no un-necessary burning shots or dev'ing unexposed film - reducing waste
it's generally cheaper, once you've got the bulk loader and re-useable film canisters and so on, in the long run, if you're shooting regularly, it'll save a bit of cash
you get to work with the same film stock over and over, and really get used to how it performs, and perfect your timings and techniques in developing the same
Downsides:
you're going to have to store the bulk loader, or at the very least the film reel somewhere cool, dark and dry - for those of us who refrigerate our film stock, finding space in the fridge for a moisture selaed bulk loader is probably harder than half a dozen rolls of 135 in a freezer box.
you've got the capital expenses of the bulk loader and the re-usable film canisters before you start.
once you commit to the bulk reel of film, you're really committed - 100feet of 35mm is a lot of frames of the same film, unless you're shooting everyday - how long does it take you to make 800 shots ?? Thats how long you're stuck with that film type.
unless you're a confirmed fan of a particular film, and only shoot that stock, you'll probably want to keep other film in stock as well - so if you've bulk loads of (say) ilford FP4, you'll quite likely still want some faster B&W and some colour C41 or E6.
Unless you're just lucky or one of those really naturally mechanically gifted people, you will screw up at some point and load the film badly, or backwards or otherwise donald-duck up - and putting right mistakes with a 100 foot reel of film, in the pitch black of a darkroom - or worse, trying to do it by feel inside a changing bag is a complete sodding nightmare.
if you're shooting modern(ish) cameras then there's the issue of DX coding - you can buy dx stickers, but it's yet another set of expenditure.
I'm not saying don't go for it, far from it, but its something that really requires going into with your eyes open.
If you shoot at least a couple of rolls of 24 frames a week (call it 6 foot of film - or a bulk roll in 4 months, barring accidents) and you pretty much ALWAYS shoot the same film stock anyway, it's a great idea. If you have rolls of film with a christmas tree on each end of the reel and a beach in the middle - not so much.