I believe Coinbase has the "don't send directly to a smart contract" guideline because Coinbase treats all addresses as one-time use, so does not monitor them for returning funds once used. So, if the smart contract gives you a refund from your transaction, it would be sent to Coinbase's address, but they don't monitor those, so your Coinbase account wouldn't get credited.

If you use a Coinbase account to directly purchase HWACCH, then the HWACCH would be owned not by you, but by the Coinbase account that sent the transaction. You then wouldn't be able to do anything with the HWACCH, since Coinbase doesn't give you any control over using those withdrawal addresses to trigger any other types of transactions afterward. So, increased gas cost probably factors into it a bit, but mostly it would be a logistically harder thing for Coinbase to do if they wanted to support users withdrawing directly to a Smart Contract like that.