i kinda agree. tahm has a kit that has gigantic potential and isnt used much for some reason.
as far as tresh goes, he really is too good at everything. compare him to leona for example. if she and her carry are ahead, shes going to be strong. she has really strong engage and can lock someone down in melee. but shes terrible in a passive lane. its difficult for her to protect herself or her carry when the lane is behind.
meanwhile thresh. he has everything leona has. strong engage and cc to lock someone up. the difference is, that he also can play passive really really easily. he can just hang back, poke with flay and pull out his carry with the lantern if they want to play safe.
most supports are usually specialized into agressive or passive play. thresh just does it all.