Thanks for this great feedback @LaurenK, and happy you have found a solution that works for you, even if this is not Smartbnb!
The reason why we are not too excited by emails is about reliability. When a message is not received, guests are lost in a street, and that exposes you to losing $$$.
Sending emails is trivial nowadays. The challenge is ensuring that emails are delivered at scale, especially as the information delivered is critical. We are sending thousands of emails to customers each day, and we can assure that emails that are not being received or delivered still is probably the cause of 10% of our customer service inquiries (and that is with a paranoid implementation, and for our own domain name)
There are many moving parts to emails, and limited assurance that the emails will be read. Even if we do a great job, it may end up in another area of your guest’s inbox that they just won’t check 
We have decided to put the priority on native messaging, because ultimately it’s the best option for you by a mile. Guests will receive notifications on their mobile app, on their mobile phone, via email, from Airbnb or HomeAway. We (Smartbnb) can check our message was delivered with 100% reliability, and have options when we know it was not delivered or sent.
Native messaging also protects you against the risk of blowback when the guest calls Airbnb to request a cancellation because they have not received check-in instructions… and you would have little to defend yourself against such a claim.
Lastly, you can add links to Dropbox or Google Docs to share a PDF on native messages, and enable you to track if the user has accessed it. We also support picture attachments; so if the document is visual enough, you can include it as a screenshot to your check-in instructions rule.