- You'll need TCPA compliant opt-ins for US sending. There's a cottage industry of people who look for vendors that aren't compliant and sue them. The fines range from $500-1500 a SMS or call. Sounds like you've already gotten that covered, but definitely go over the requirements with a lawyer that specializes in it.
- SMS is throttled per a phone number. This is throttled at both the provider and the carrier. That means that even if Twilio sends something at 10am it might not be sent by Verizon until 11am. This absolutely impacts off-hour SMS sending.
- SMS is pricier than email and even at the highest volumes has a relatively high floor per a text. I think the floor might be $0.004X-$0.005. This is offset by having better response rates that email. And as others have mentioned the providers will give you better pricing as your volumes go up, but you'll need to talk to account reps about that.
- At a certain size (1M+/month?), you can also consider integrating directly with the carriers, although that's a longer process to get set up.
- You'll need to manage SMS bounce/spam rates like you would for email bounce spam rates. You'll end up maintaining a block list of known bad phone numbers
Edited for formatting