Ok, fair enough...I don't have an adapter (although, I do have an FD 50mm F1.2 that I'd like to use...so I might try one someday).
One of the issues (maybe a better word than hassle) is that (from everything I've heard & read) you have to choose between losing infinity focus (adapter without lens element) or degraded image quality (adapter with lens element). The amount of image degradation is up for debate, of course.
Another issue is that most EOS cameras (especially the entry level DSLR bodies) aren't really designed for manual focus....the viewfinders are rather small and dim compared to older cameras. And they certainly don't have split prism screens like the manual focus cameras. You can put them into some modern cameras...but that's another issue.
There is the issue of stop down metering...and other metering variances. Probably not a huge issue...but certainly things that aren't an issue if one just used EF lenses.
One of OP's main concerns, per his actual words, is budget. If money's no issue, sure, order your favorite EF lenses to go with that spanking new body. But if money is an issue, and you ALREADY HAVE FD lenses from shooting Canon SLRs for 35 years (again - that whole "actual words used by the OP" thing), then FD to EF adapters are a great idea.
Here's more of OP's actual words in use: He mostly shoots macro of plants. Guess what? FD to EF adapters work great for that since (get this) you don't NEED infinity focus for that sort of thing and that means you don't need to keep the adapter's lens in for that kind of shooting. If he ever does need to focus on infinity, he can put that lens back in place to do so.
Oh, and in case you missed it, the lens in question can be removed and replaced in the adapter, at will.
And let's talk about that "degraded image quality" you brought up with the adapter's lens when it IS in place (again, won't be an issue when he's shooting macro, which is "most" of the time per his actual words, because it won't be IN the adapter when he's shooting macro). Nonetheless, here's a link to images I shot with an FD to EF adapter WITH the lens in place in it:
http://www.thephotoforum.com/forum/nature-wildlife/294261-critter-shot-500mm-f-4-5-l-fd-mount.html
Do you
REALLY see image quality issues by having that lens in place? Especially for the cost savings of buying an adapter vs buying new EF lenses when on a tight budget? Let's be honest. Sure, if there's another piece of glass in the light path, it will affect the image. But to what degree? In what way? Is it perceptible? Is it detrimental? Are the images unusable? Will anyone even know if you don't tell them? Seriously now. Citing this as though it's a serious concern is just silly.
The viewfinders are small and dim. OMG. They're not so small and dim that you can't see what the heck you're doing with them. Are they the optimal viewfinders of their big brothers or older cameras? No. Does it REALLY matter? Not much. And, as you mentioned, replacement viewfinders with split prisms and brighter screens are available. You forgot to mention that they're a LOT less expensive than a new EF lens as well. (Budget-minded OP, remember?)
Metering variances are SUCH a HUGE problem when shooting macro and chimping immediately and having a histogram and "blinky's" to see if you've got the right exposure before firing off the next 100 frames, right? NOT. He could even shoot "Live View" and have a pretty good idea how things are going to turn out exposure-wise. Welcome to the digital age of photography.
Try this: Get an adapter and get some hands on actual experience. THEN use that actual experience as a basis to advise others when you get the urge.
There are just as many "hassles" and "issues" with using reversed and/or stacked lenses, extension tubes, bellows and so on, but you don't chime in to advise macro shooters against using them because of the "hassles" or "issues", do you?
We all use what our budgets allow us to use. If he's got FD lenses from his 35 years of Canon SLR use, and not much budget to work with, and mostly shoots macro, then I think your advice about FD to EF adapters is questionable, especially knowing what I know about what it's like to work with them, and how well they actually work when you do.