Of course, email is from the 1970s and does not natively support emoji reactions. That makes this a Gmail-proprietary feature, which is a problem for federated emails that are expected to work with a million different clients and providers. If you send an emoji reaction and someone on the email chain is not using an official Gmail client, they will get a new, additional email containing your singular reactive emoji. Google is not messing with the email standard, so people not using Gmail will be the most affected.
Another weird quirk is that because emoji reactions are just emails (that Gmail sends to other clients and hides for itself), any emoji reactions you send can’t be taken back. There’s only Gmail’s “Undo send” feature for taking back reactions, which delays sending emails for about 30 seconds, so you can second-guess yourself. After that, you’re creating a permanent emoji reaction paper trail. […] If the idea of emoji reactions to email has you selecting the puke emoji, as far as we can tell, there’s no way to just turn this off. The report notes that this new feature won’t work on business or school accounts. “Emoji reactions also aren’t available for group email lists, messages with more than 20 recipients, emails on which you’re BCC’d, encrypted emails, and emails where the sender has a custom reply-to address.”