We have been working with person accounts in our SF setup till now and therefore using just contact Id as the subscriber key during email sends. Now we also have a requirement to send emails to business accounts which do not have a contact Id. In that case, is it okay to use Account id as the subscriber key or would it have any implications? As in having both account and contact ids as subscriber keys in Marketing cloud?
Not something you should be doing unless you want to deal with some inconsistencies and some features becoming unuseable without necessity. There are known issues with it today, and it also is kind of risky as the software progresses, because SFMC makes some core assumptions and is developed along those lines.
One of those is: "Persons" for Marketing Cloud that can be injected into journeys can only be leads, contacts and users. So your business accounts should also have at least one "Contact" and that's the person you inject.
Journey Builder does not consider AccountIds "Persons" that are injected into a Journey and are supported as such. For example the whole built-in integration of Journeys and campaigns from MC Connect won't work. (You can't add an account into a campaign without journey builder either, so that makes sense). Sending back tracking to SFSC (if you really want to do that) through the connector also doesn't work with Accounts, but you get it out of the box with MC Connect.
The way the system's features are intended to work is using the PersonContactId on the Account object in the case of PersonAccounts, and the ContactId of a contact under normal business accounts.
Those two both start with 003 (as they are technically the same thing at heart). Avoid the AccountId that starts with 001 as a handle for your persons.