The Irn-Bru First Division champions have taken to life in the Clydesdale Bank Premier League with ease.
Despite having no previous experience in the top flight, they have clinched a top-six slot and are still in the hunt for a Europa League place, five points behind Highland rivals Inverness.
But Tannadice midfielder Armstrong, who will face Derek Adams' side with his United colleagues in Dingwall on Friday night, expected to earn more than just a goalless draw in the teams' first meeting back in September - until County's 11-match unbeaten run that stretched from December to March forced him to reappraise their talents.
He said: "I've been surprised by Ross County, to be honest. I remember we played them earlier on in the season and being disappointed with a 0-0 draw.
"But they have shot up the league and put in some performances that have merited such a high position so it will be a tough game and they have got some good individual players.
"What their manager has achieved has been a phenomenal achievement. He's obviously doing something right up there and he's got his tactics spot on. Their season has been very surprising but one they can be very proud of."
United still have an outside chance of beating Caley Thistle to the final European place themselves. But with seven points to make up and just four games remaining, time is running out.
Last week's 3-1 home defeat to Motherwell hardly helped their cause and Armstrong says United must learn from the poor start that saw them two down inside 10 minutes.
"We've had games this season where we have started disappointingly but that's something we are trying to cut out," said the player who was this week handed a nomination for the Cheque Centre PFA Scotland young player of the year award. "We need to try to control the game from back to front a bit more and not allow these easy goals to be conceded."