Shedinja

I mean, rather use Venomoth. It has way too low attack stat that holds it back even it gets Wonder Guard.

Only have 2 moves per pokemon and extremely limited movepools. Some pokemon (and likely many) not having access to moves from the 5/18 types that could touch wonderguard shedinja.

Amped Shedinja would completely change the defensive tier list eg. it would make a punching bag out of present blissey.

Even with Wonder Guard, Shedinja would still just be a novelty Mon. It's base stats are just too low for it to ever be a serious competitor. That said, here are my thoughts on it as an attacker/defender if it had Wonder Guard.

Attacker - essentially no practical use whatsoever due to its pitifully low attack stat. Even in instances where it would take no damage from the opposing Mon's moves, its own damage output would be laughably low. There would likely be some instances though of players putting one in the first slot of certain raid lineups if they're in a large enough group (so the individual damage bonus is more or less meaningless) and they don't want to use potions/revives.

Defender - it would certainly function as a unique defender, I'll give it that. It would force the attacker to slot in a Mon that has access to one of the few move types Shedinja is weak to. Inexperienced players who don't plan accordingly could face an unbeatable Shedinja (and potentially think the game is lagging out when they don't see Shedinja's HP bar moving). Against a knowledgeable player, however, Shedinja would just end up fainting in about a second. I could see Wonder Guard Shedinja getting its own special tier ranking due to these reasons as it's either unbeatable or gets one-shotted.

It would never be a good attacker cause it's slow. It would never be a good defender cause you choose your Pokemon when attacking.

Could potentially be an anchor vs some raid bosses so you don't need to rejoin (or to stay in combat so the boss doesn't regenerate till others rejoin)

Not at all.