Haven't played it yet, but Mr Bigglesworth doesn't seem very good.
* It does nothing until someone dies. Because you're essentially playing a powerless hero up until that point, that's more likely to be you than literally any other hero, as even the worst hero that actually has a power has some advantage over a completely vanilla character.
* Even when it does do something, it's a crapshoot. Getting a ready made minion is definitely good, but not only is it pretty random just how good that minion is, but it may not have any synergy with your own warband.
* With the previous point, the lack of synergy might be fine if you end up getting something with enough stats to include anyway - but the people who die earlier are more likely to have weaker minions with less buffs.
I honestly think it will end up in tier 4. Not having a hero power until the mid/late game is pretty crippling, and it makes you rely more on lucky tavern rolls than basically any other hero - then on top of tavern roll luck, you're hoping on discovered minion luck when your hero power finally does go off.
Edit: apparently keeping enchants also means staying golden (if it was), and apparently when you play said golden minion, you also get a triple treat. That could be enough to make the hero viable. It's still uncomfortable playing a powerless hero until the midgame at earliest, but if you top 4 you're probably getting 2-3 free golden minions off the hero power, which makes it end up being as good as Eudora (though slightly weaker in that you're not getting the first by turn 5).