Most Tahm's use him incorrectly, particularly his W. The only Tahm I've played who genuinely impressed used his W to negate a fed diver or used it offensively to delivery their biggest threat right into us to start the fight. He also frequently used W as an AoE skillshot, essentially running up to us, slap us with Q, grab a minion and spit it at whomever he landed Q on. Very effective lane pressure.
If you're in the teamfight stage and your mispositioned carry gets bursted down and you have to blow your W for the save, odds are he's dead anyways. I get why every Tahm feels obliged to save them though, the amount of whinning "Tahm no W me report nonsense" would eventually get to anyone. Tahm's W is just like Guardian Angel in that way. If you suck at positioning GA's worthless, ditto Tahm's W.
Tahm has a 95% slow if he devours so he can't deliver their biggest threat "right into us to start the fight", instead being barely able to move at all while giving the enemy free invincibility. Plus, because 3 stacks plus devour, even if he instantly lands a Q and an autoattack, he can't do it at the start of the fight. He has to be able to sit melee range long enough for two autoattacks, or have a fight dance going on for 5+ seconds.
Q can be negated by any minion, similar to Blitzcrank grab, so I'm not convinced it can give reliable pressure. W spit likewise stops on any contact, plus it means you're defensively weak while it's down.
Now I'm saying all of the above as a bronze/silver player so obviously my opinions have caveats, but I do think there's been some a reasonable breakdown of why Tahm's kit isn't very good in general, at least with the numbers right now. I can try to dig it up if anyone wants.
Another issue is that a lot of players may not even realize how his speedboost and teammate devour works. You can do things like speedboost a jungler into lane, but that (for example) 40% speedboost attacking or 20% speedboost running away is lost for both players if someone randomly clicks and spits out in 2 seconds instead of 4. Plus, the random clicking will probably make them spit out in the wrong direction, potentially killing them instantly and wasting the entire save and escape sequence.