A United Ireland would merely recreate the exact same issues that may have arisen after Brexit. While Northern Ireland did vote to remain in the EU, a large portion - especially among Unionists - voted to leave and would not accept being forced to remain in the EU after believing they had won the referendum.
While cross-border Irish interactions may be easier, cross border interactions between Northern Ireland and Great Britain would be more difficult. Many in Northern Ireland do business or have family in Great Britain, and a United Ireland after Brexit would suddenly present the same type of border problems that a North-South hard border would create.
As for a hard border causing violence, a United Ireland would surely be even more likely to cause violence and potentially restart The Troubles.