Mastodon caches images from remote instances, Lemmy doesn’t. It links, which has it’s own issues (images disappearing from remote posts if the instance goes down). Everybody I’ve seen complaining about storage space have been open Lemmy instances with loads of users potentially uploading images to the pictrs sub-system. There’s even been requests to limit image storage for individual users.
If you’re thinking of using the YunoHost version of Lemmy, be aware that the pictrs subsystem is removed due to technical issues, I would absolutely recommend using the ansible install. But that might defeat the purpose of easy self-hosting for most YunoHost users…
From a non-technical aspect, the biggest issue (or at least used to be) on Lemmy was the ideological attacks and isolation by the fascist core community of any non-conforming instance. That had a chokehold on the lemmyverse to the point I saw no point running a Lemmy instance unless I self-censored my sometimes negative opinions of the Chinese government. Seeing the core developers being willing to hold back their own project was the biggest surprise when self-hosting Lemmy.
Lemmy is created by what many describes as “tankies”, fundamentalist communists. The lead devs are, in my impression generally supportive of the Chinese government. They run their own instances, dedicated to their own world view and interests. I don’t have the slightest problem with that.
Hopefully due to a lack of foresight their instance was presented as a flagship instance on the official landing page and other places, effectively making it the representative for Lemmy for a long time. With their moderation practices, they in large parts drove away what I consider reasonable users regardless of personal ideology.
This created an monolithic instance that dwarfed every single instance besides one; the other dev-run instance which seems like a containment instance for those too fundamentalist to stay on the “main” server. For a large part Lemmy was a downright nasty monoculture that would harass even lead developers for daring to interact with code-contributing users on instances deemed “problematic”.
With a very generous blocking practice, these monolithic instance basically shut out anybody not agreeing with them from the majority of content - including non-political. This in turn gave other instances less of a reason reason to stay on the Lemmyverse which reinforced the monoculture.
The devs did take steps to rectify this problem: they offered a year of free Lemmy hosting for anybody willing to admin a general purpose instance, recruited other general purpose instances to promote on join-lemmy.org. I respect that. But at that time it seemed too late to change the direction. When I realized the bad reputation Lemmy as a project had amongst some of the larger fediverse influencers, I realized that Lemmy would not be viable until it got a massive and sudden influx of new users to shake up the overall federated lemmy community. And now here we are!