It is true that emoji reactions (picture stamps) are interesting and useful, but there are also some problems with this system.

1) Picture reactions can be put instantly, and a lot of stamps may be unfavorable pressure. It hasn't caused troubles in mastodon so far, but misskey users must undergo them.

2) Pictures may add or direct some specific, unintended meaning to the post. For example, a serious text with funny pictures may look insincere. Fortunately, this worry remains a potential one currently. Furthermore, it may cause inter-cultural conflicts.

3) Offensive pictures can be attached from other users (of different servers). The structure of mastodon is open to the risk. We also have countermeasures, but administrators may have to bear the burden of screening.
(cont.)

(cont.)
4) Picture reactions are one-way, that is, the poster cannot react, refute, query, refuse, delete, the reactions. When it comes to some problematic subject, numerous reactions may result in asymmetric attacks. I hope such circumstances have not happened yet, but as the population increases and various kinds of people are joining in mastodon, the risk may arise.

5) *(maybe Japan-specific?): Japanese servers such as misskey and fedibird offer a lot of original pictures for reactions, but some of them are just emoji, that is, "picturized words" in Japanese language and often otaku (geek) -oriented. Pictures with specific language is against the idea of universal design, whatever language they are. And (cont.)

(cont.) And some local custom and memes may be imposed on others. Again, I am worried about the intercultural frictions and conflicts caused by picture reactions.

The arguments above are based on the discussions among Japanese users of mastodon and misskey, which has been talked for years.
I am not sure whether those anxieties apply to other servers globally, but I think we can consider the risk of the function, though they are truly interesting and now I love to use them.

@mikoto is this something mastodon's looking to implement? :o i've not heard much news on it if so

i love the feature on misskey, but not being able to remove a reaction from your post feels like a basic oversight... i can imagine a lot of rude folks adding a clown emoji to attack a post. i'd hope that gets addressed before we see it on mastodon/elsewhere

@niaco My posts are the reaction to someone in my timeline who wants mastodon.social to introduce them.
For the present we are enjoying emoji reactions among well-moderated small servers peacefully, but if, if ever, the largest server are to implement this function, I can't expect what will happen, though I hope the users are thoughtful enough.

@aetios @niaco Yes! Urusai server, in which I am posting, kindly offers almost 1,000 emojis (so many that I can't count exactly).
Whether to accept emoji reactions are up to the judgements of the administrators for individual instances, and I think it is a good way.

@aetios Now I sent a emoji response from urusai server to your post.
Do you receive the picture of a blue haired girl raising her left hand?

@aetios @mikoto Oi I did not know urusai supported so thats cool to know.