The answer is simple, he is the Hokage we know the least and the few feats we know about him are overlooked because:
1 - His water jutsu
When Tobirama made his debut in the manga the fact he was able to use Suiton techniques with no nearby water bodies was treated as a legendary skill and an statement of his power, even Zabuza needed water. Today, many water users displayed the ability to vomit their own water making Tobirama's once legendary skill something mundane. Yes, you heard me well, Tobirama vomits water to make his Suiton jutsu, the air moisture stuff was only in the anime.
2 - Edo Tensei
Indeed one of the most broken techniques ever created in this manga. We know Tobirama created it and that alone could make him one of the most powerful characters, except for the fact it was Orochimaru who finished it. That makes the implication he started researching it, but changed his mind, labeled it as forbidden and dropped the project. It is questionable how far he came into said research or if he was able to use Edo Tensei in an incomplete form.
3 - His S/T Technique
Probably the most controversal of his skills. Kishimoto said he had a S/T jutsu, but never stated how it worked, its effects, we don't even know the name of the jutsu. It is problematic to take into account a technique we don't know anything about.
By the way, there's another problem. He is the brother of the legendary Hashirama, a character that received so much attention nowadays his feats are completely overshadowing his brother. People tend to overlook Tobirama in favor of Hashirama.
Well, I suppose it explains the most common reasons about why he is so underrated. Basically, people don't want the author to say "look, that character is strong" through the mouth of someone to prove said character's strength, they want him to actually show it. We had an opportunity to see Hashirama's techniques through Madara's hands, now we need an opportunity for Tobirama, an actual display/explanation of his S/T tech should help.