Here’s a list I made last time I tried it. Might not all still be true!

• Signal accounts are tied to a phone number, which is dumb and limiting. You can’t contact someone by username only (or let someone contact you without giving them your phone number), nor can you have multiple phone numbers (I have a work phone and personal phone).
• Signal has no polls or pinned messages.
• No cloud chats comes with a few disadvantages e.g. no global search across all chat history.
• No cloud media storage means you only can keep as much as you can fit on your device, whereas in telegram i can still look at pictures sent to me a decade ago.
• No bot api, which most people probably don’t care about but sucks for me cos I regularly make telegram bots to improve my experience and for fun.
• Signal also has no custom themes but that’s kinda meh.
• Signal has no silent or scheduled messages, and editing/deleting a messages is time-limited (and beta).
• Pretty sure telegram has better emoji reactions and animated gifs etc though not sure.