Replying by email within an SMS ticket
A Zendesk support agent has confirmed for me that it is not possible to send reply by email from within an SMS ticket.
This function would be highly desirable for the purposes of keeping all communication about a single issue within one ticket.
Losing reply options after merging email tickets into SMS tickets, and vice versa
Current functionality also makes it so that if you merge an email ticket into an SMS ticket, the agent loses the ability to send emails from within that ticket. The inverse is also true for merging an SMS ticket into an email ticket.
Preserving all of the communication options would significantly reduce proliferation of tickets for a single issue.