I think I look at it the other way. Newbs are correct in being afraid to make their own because they have no idea what they are doing yet. After all, one doesn't learn to drive by learning to make gasoline, right? ;-)
I'm not sure how prevalent borates are in the chems you are referring to (any specific example?) and I'm not sure how big a deal it is either, unless you happen to be "one of those people" who never do water changes. (Bad for newbs!) I will say that borate salts are the most common "contaminant" in DIY-grade Calcium Chloride (which I would also not recommend for a newb) and that doesn't slow those of us down who're using that.
Also on some of these "mystery ingredients" you're looking at one manufacturer who lists the known impurities and others who just don't say anything but it's still in there. Pickling lime (kalk) is a good example for this. Mg and Sr are common impurities in calcium hydroxide, but only one manufacturer puts them on the label in any fashion. All this means is that (surprise!) it's safe to assume that all reagents have some degree of impurity.
You have to know what's significant about the impurities (if anything), not just what's in there. Outside of speculation I've never heard of a tank problem being associated to borates in actual reality.
IMO, a commercial product is the right way for "most people" to start out. Specifically I recommend Brightwell's Alkalin8.3 and Calcion. Their instructions and other detailed information on the bottle is the best because it is conducive to learning how to test and dose
and understand what you are doing. Learning these things should
absolutely be the name of the game for any newb getting into stony coral. And while online is a great resource, it's hairy trying to learn about dosing online for a newb. (Look at the unfortunate results so many people end up with at first.) FWIW, the best alternate brand (IMO) has no usage info at all - just the concentration data (which at least enables proper usage). Most of the rest have advice that is actually bad/harmful on the label. Not good at all for a newb, even if the ingredients are great.
Anyway, by the time the extra cost of the premixed reagents begin to be noticeable, the testing and dosing routine should be "old hat" and there will be enough understanding not to "be afraid" of mixing one's own dosing reagents. If you wanna bother with the mess - there are always the possibilities of waste when you mix your own and spoilage any time you buy in bulk - this would be the time.
Personally I recommend adding kalk to your dosing routine and maxing our your water change schedule to cut your 2-part dosing down significantly prior to getting into DIY reagents. It's just a good way to do it.
-Matt
P.S. In case it helps anyone to know: I use anhydrous calcium chloride (road salt from the wholesale club) and Arm & Hammer as my two part. There is a orangy-red precipitate when I mix my 5 gallons of Calcium that I suspect (based on some reading) is some kind of borate. I do take measures to dose almost none of the precipitate, but I do nothing to remove it from the dosing reservoir. I do "magnesium maintenance" primarily through not-often-enough 20% water changes with Reef Crystals and a very occasional "bump up" using BRS Mag supplement. I would never start a newb out doing what I'm doing even though it's very inexpensive.
The point of the gasoline example was that the process of making gasoline is wholly unrelated to the process of driving a car. So, I would never recommend a newb driver to get involved with making gasoline as a part of their learning to drive. (Even if it would save them money and even if it were not very difficult.) Likewise, the process of handling, mixing, etc doesn't help (and may complicate) the learning of testing/dosing and what the chemicals being dosed are all about. All in good time - first things first. That's all!
