He wasn't as serious a threat as the Teutons, cimbri or Hannibal (though I guess that is how one interprets a threat). Lets look at the three Mithridatic wars and see how much of a threat he posed in each one.

First War: Mithridates was king of Pontus and was surrounded by Roman client states . Mithridates after expanding his kingdom to the black sea coast decided to expand into the kingdom of Cappadocia, at the time ruled by Mithridates' sister and her husband. Mithridates had his brother in law assassinated leaving his sister to rule. Laodice ( Mithridates' sister) however married the king of Bithynia, Nicomedes the third (Mithridates' enemy). After some minor intrigue Mithridates killed his nephew and claimed the kingdom of Cappadocia. Nicomedes appeared before the Roman senate and demanded the Romans remove Mithridates.

So they sent out a legion to restore order. Mithridates appeared compliant but soon took back all his territory and began a massacre of all Romans and Italians living in the Asian provinces he controlled. Mithridates captured all of Asia minor and established himself in Greece, even setting up a puppet in Athens. The Roman consul Sulla responded by invading Greece, most of Greece switched allegiance to Rome but Athens remained loyal to Mithridates, this prompted Sulla to take and thoroughly sack the city. Facing military defeat and rebellion Mithridates made peace that left him in control of Pontus and nothing else.

Second War: In comparison to the first war the second war was a minor event. Sulla had gone back to deal with trouble in Rome and left a general named Licinius Murena to deal with affairs in Asia. Murena invaded Pontus claiming that Mithridates was planning an attack and rearming. Mithridates inflicted a minor defeat on Murena and peace was restored.

Third War: Nicomedes IV died and left his kingdom of Bithynia to Rome. Mithridates took his chance and attacked. The Roman general in charge this time was Lucullus, he didn't do a bad job, but he was unable to decisively defeat Mithridates (and his troops mutinied, which hurt his authority) and so the Senate was forced to send Pompey. By this time Mithridates had lost much of Pontus and had been forced to retreat to his Son in law's kingdom in Armenia. Pompey marched into Armenia and defeated the Armenians and Mithridates. The Armenians sued for peace and became a Roman client state, while Mithridates fled into the very northern part of his kingdom. Pompey pursued him, this combined with Mithridates second son leading a revolt caused Mithridates to kill him self. Legend has it he tried to poison himself but couldn't because he was immune, so he had a bodyguard stab him.

So overall was he a threat? He certainly wasn't going to march on Rome but he was a skilled military commander who was seen as someone capable of defeating Roman armies, this made him a threat.

Sources: Rome and Her enemies: An Empire Created and Destroyed by War by : Jane Penrose

The Poison King: The Life and Legend of Mithradates, Rome's Deadliest Enemy by: Adrienne Mayor