Trey,
I'm sure you think me the garlic nut, but it's one of the few things I have enough experience with to give qualified advice on... so, here goes. Downsides # 2, 3, 4 and 5 can probably be attributed to using oil-based vs. water-based extract. Having used both numerous times on my tanks and those of my friends', I've witnessed these symptoms with oil-based extract (i.e. squeezing gel-caps). Whith a nice water-based extract, however, you avoid these problems. I suspect water-based extract does, however, have it's own downside in that the garlic may become dilluted more quickly in seawater. To counter this, I try to use creative means to ensure that the fish gobble up the garlic-infused food quickly (i.e. soaking live brine in garlic extract).
The extra work and the need to continue treatment for a month do, of course, come with the territory. I've tried to alleviate some of the extra work by doing things like thawing Formula 1 for a bit... soaking it in garlic and re-freezing. I've done this for friends as "medication kits" and it's apparently worked. The caveat, again, is that they need to feed it in doses small enough for the fish to eat it ASAP. I suspect oil-based extract is less prone to dillution.
Just my observations???