This is a slightly biased answer, but I'm going to provide a through explanation how I feel that Regieleki IS
viable, but somewhat underwhelming if not considering tera typing.
Regieleki possesses strong electric stab, while being one of the best, if not the best pivot in the game due to its sheer speed. However, one common trait of some Pokemon in the Ubers/AG metagame puts Eleki completely at risk. That is its over-reliance on its stabs.
Eleki has no other moves to provide coverage except tera blast. With defensive titans like Eternatus, the ever so prominent rapid spinner and offensive ones like Miraidon in the metagame, Regieleki can have a hard time trying to break through these staples of the metagame, meaning, if has to waste the team's one singular tera, to change its tera typing to ice and to give it an edge over some Pokemon.
This, however, is able to be reversed with careful teambuilding. Many players do not choose to have Regieleki as one of their cores in a team, but with checks towards the common answers to regieleki(answers: miraidon, eternatus, groudon, amoonguss, big tusky [he got 2 very large horns]), Regieleki is able to perform favorably.
In conclusion after all that rambling and random fancy words, Regieleki is a pretty nice Pokemon, as it can volt switch out of most targets and run. But this is only accomplished with careful teambuilding and blah blah blah hyper offense.
