(caveat - I'm a bit out of touch with the exams these days so my comments are applicable to some years ago and may not be so now)
(a) problem with DIY is that the recommended syllabus and texts may have a remarkable dissimilarity to the exams .. eg, as an undergrad in the late 60s, I thought to knock over the ATPL exams in Oz .. ANO (now CAO) 40 had a nice syllabus listing, gave some recommended texts (which I duly sourced and studied) .. so far so good.
Came exam period and the exams bore scant resemblance to any of the above .. I probably got a couple of marks for spelling my name correctly. At that stage I hadn't heard about "past papers". A review of these and the next sitting saw the various exams knocked over without any trouble.
So, if you have a half decent secondary background, study the usual texts, do a bit of past paper practice to get a handle on the exams, then you should be able to knock them over without too much trouble.
(b) the advantage of sitting in on a good formal course (or doing a good correspondence equivalent) is that you can save a fair bit of time.
Caveat is that some providers get carried away with past papers and teach to the exam, not the subject. Result is that you might pass the exam but end up knowing little about the subject .. and this latter requirement is what the exam was there for in the first place .. so beware of the provider quality.
Going back some years, there were several suitable providers in the Melbourne area .. can't comment now as I am a bit out of touch with this side of the game .. did my time teaching such things to newbies .. many of whom are now floating around as Chief Pilots etc., here and there.
Main thing is ... get the subjects done as, otherwise, lack of the passes will provide a brick wall to your progress sooner or later.