When you are pinged do you check the location of the ping or the person who pinged it?
Most people i wager check the location of what was pinged and since tahms ult pings where hes going and doen't have any indication that hes ulting most people tend not to see it. This is different in a game where you are using voice chat but since in solo que people might not be its a big problem. The last few games i was playing people walked right past me while i was sitting there waiting frantically pinging them to get in my belly and they would take around 5-8 seconds to finally realize what was going on.
Its a problem and it has nothing to do with trust and more to do with readability for Tahm Kench allies.