It gets a bad rap because it destroys so many peoples guts lol. I can't even take 500mg of the stuff without the worst stomach pain of my life, many people I know are the same, not including DXM cough syrup but the extra strength Mucinex tablets. They are well known for causing digestive issues, you put it together with DXM and it only adds more stomach problems to a substance already commonly attributed to them (at least the syrup, I've never done extraction or the pure DXM tablets). I've known people who took it with DXM and were fine so everyone's different. Back when they had just DXM syrup which you can't really find in my area, we'd tell everyone to avoid the green bottles because of the guaifensein just based off bad trip reports, but at least 5 of my friends got these and they were fine.
I definitely wouldn't suggest anyone even attempt to take a dose this high without at least seeing how their stomach does on lower amounts. Even then, I don't think it's worth abusing for these reasons. You have an iron stomach if you can take doses that high. Hell I have an iron stomach, used to go through 30+ shots without any nausea, only ever thrown up once from drinking which broke a 12 year spree without puking, but any amount of this stuff will fuck me up, bad. My GP recommended it for fluid in the ear last Fall and I could not believe the absolutely severe stomach cramps, probably the worst I've ever had. It was like my guts were fighting this stuff and losing the battle, and then insane constipation for days, and trust me I drank tons of water anticipating it drying me out, I always drink a lot of water since I don't drink anything else anymore.
Anyways, my underrated drug would be Ritalin, probably. I find it way more euphoric than Adderall (which I find personally just makes me feel uneasy and not myself). Great for making me social, confident and energetic. I remember it being referred to as "cocaine with a PG rating" in House lol, and I have heard it's high is more similar to cocaine than some other stimulants.