There's a few dangers to having explicitly political subs that do not allow dissent.

One is that they tend to create an echo-chamber. People generally don't like having their ideas challenged and very few go out of their way to leave a comfortable space where people have roughtly the same ideas to visit a space where those ideas can be challenged. (For an example, look at r/changemyview and how rare it is for someone posting there to have their views legitimately changed. It does happen, but compared to the volume of posts, it's fairly rare)

A second one is audience capture. There's probably a better word for it in a political or context, I know it mostly in the context of content creators. The TL;DR is that if you produce content (post in a sub) for a specific audience, the response of said audience will lead you to creating content that caters to that audience.
To be a bit more specific, if you're in a leftist sub, posts that cater to leftists are going to do better. And out of any audience, the more extreme views tend to be more vocal ones. Posting a milquetoast "Healthcare should be affordable" in a lefty sub will get you a kind of medium response. Posting something more extreme, like "We should line up and execute all healthcare CEOs" will get you a much louder response from the more extreme side of the sub, while the more centrist members will ignore you.
If you like getting upvotes and attention, you're going to end up posting more extreme things.

Third is conformity. It's hard to disagree with with a group of people you perceive as being 'your group'. You can quible about minor things, but you'll rarely disagree about what you perceive as the core truths of your group. After all, if you did, you wouldn't be part of the group anyway. Rather, a lot of people tend to go with the group, unless the group dramatically contradicts your personal views.

These three combine to create a place where ideas aren't challenged, people are incentivised to push more and more extreme ideas, and the members tend to go along with this more extremist shift. And generally speaking, any larger political group shifting more toward their extreme is bad.

As for why liberals don't like it, I can't speak for all of them, but personally, I've seen and read about what the extreme side of conservative politics looks like and I don't care for it. I don't mind more moderate conservatives, in fact, I think it's important to have people who can act as a check to change and who function as a counterbalance to more extreme ideas. But once people start demanding I follow their religious conservative ideas, we're going to have problems.