It does not for Tan. He's the event character and scales with the upgrade levels. For all the other characters it matters, but only when you get to the relevant wave. For instance, if your Isabella is Bronze 3, she will only be weaker than possible as soon as you get to the "Rare" rank. She will be capped at Stone 1 for common, then Bronze 1 for uncommon and then Bronze 3 for rare. If you get to epic or legendary, she will stay at Bronze 3 or be dead a long time a ago most likely.
66995 with G3 Actus, D2 Calgar, D2 Bellator, G1 Vindicta. Getting multiple waves of Bellababies is great fun.
Did 70100 with D3 Aсtus, Calgar, Thadd, D2.5 Bellator
Does it make a difference if your units are low ranked? As in you have not build up the characters yet but use them here?
It does not for Tan. He's the event character and scales with the upgrade levels. For all the other characters it matters, but only when you get to the relevant wave. For instance, if your Isabella is Bronze 3, she will only be weaker than possible as soon as you get to the "Rare" rank. She will be capped at Stone 1 for common, then Bronze 1 for uncommon and then Bronze 3 for rare. If you get to epic or legendary, she will stay at Bronze 3 or be dead a long time a ago most likely.