A like system would probably be okay, I don't see any major drawbacks, and maybe as a result of having a like button we will see less users giving merit because they agree with a post. Since, in my opinion a like system is completely different to the current merit system, which sometimes gets treated as the same by certain users.
A like system without limits on the number of likes that can be sent would result in people using sockpuppets giving likes to their alts. Look at
this thread, someone has many accounts that were always logged in, why would you think that someone wouldn’t use zero post accounts to give many likes?
No doubt that is still abusable, but I think the likes wouldn't carry as much weight as the Merits though.
I mean, the person could always use sockpuppets but in the end the BS would still be smelled out by other members sooner or later.