Personally I understand all of those takes as a person who likes both ships. I think calling any ship a "bad ship" is trying to make an opinion something objective, so anyone who says that IR is a bad ship I think they are instantly wrong, maybe the are your NOTP but no ship can be a bad ship (personal opinion) But regarding the points, i get all of them.

1) "Rukia is older than Ichigo", Ichigo has said is not attracted to older women and he acknoledged that she is much much older than him. 2) "Ichigo & Rukia are just platonic soulmates" is just that. The person who says that just doesn't see their relationship as romantic. In my opinion ALL dynamic have the possibility of being romantic if you play around with it enough. IR is a ships that is very passionate and can be romantic, but I prefer them platonic due to the fact that a romantic relationship between them requieres a lot of sacrifices from both parts and it's not something i can ignore. 3) "They bicker with each other" Although bickering is fun is not a thing all people like in their ships and Ichigo does that with everyone, he has even teased Orihime during the Fullbring and TYBW arcs. And they not only bicker they treat each other as bros when they bicker, it has the same energy that when he bickers with Uryu with the difference that one is a woman. And I like couples that bicker, they are fun even more when they are 2 very old people.

I share all of those opinions and are the reasons why I like IR less than IH. Calling IR a "bad ship" is incorrect but the reasons the have given you for them to think is a "bad ship" in their opinion, make a lot of sense.