I have nothing to back this up, but suspensions seem like they're likely to result in wildly different doses no matter how well shaken. I get the appeal and ease of making it., but the idea that after shaking, you take a second to pull a random drop, and every time you do that the drop will have even just 20% +/- the dose of another drop is hard for me to wrap my head around.
The other issue is pharmokinetics.
Drugs are designed and tested, and dose determined with the idea that they're absorbed at a certain speed, in certain places.
The guy taking what he thinks is 5mg of minoxidil from a topical drop orally, is absorbing that all at once on his tongue, then says "this stuff is crap, it sent my heart racing and my head bloated up so much skin was sagging" isn't considering the tablet slowly absorbs over 40 minutes in the gut, and only 2mg effectively enters the bloodstream, for instance.
It can be even more complicated with other compounds.
Take metformin as a suspension and you'll never repeat it, since it destroys your sense of taste for an extended period leaving metal in your mouth. Stick it in a capsule, and when it's absorbed in your stomach you'll get all the worst side effects, and almost none of the benefits. The tabs are designed to deliver metformin mostly to the small intestine where it does th most good.