I think Juulsen has a high end defensive game but in the NHL his lack of mobility has been an issue for him.
As for his offensive game, the problem there isn't so much his lack of offense as he does have a big shot and can move the puck, the problem is he hasn't progressed there since his draft season. In his draft year he put up 52 pts, if you look at the WHL's all time points for U-18 defensemen since say the past 15 years he would be tied for 12th OA.
Elite Prospects - WHL Stats All-time season
It's not to say he's got high offensive upside, just that he was trending in the right direction. In his draft year his team was one of the top teams in the dub and they were 9th OA in goals for as that's the year they had Scherbak leading the team in points by a wide margin. Juulsen led the team in points for blueliners and was 5th OA in points on the team.
The next year his production took a nose dive going from a hair under a .77 ppg to a hair under .45. But his team lost Scherbak and the offense dropped to the bottom of the league as only 1 team in the WHL scored less goals. That said you hate to see such a big drop in production. The next year the team was once again a top team but this time around it was due to the defense and goaltending as the offense was ranked 15th out of 22 teams. Juulsen did rebound a bit with a .69 ppg which if he had played the 68 games like he did in his draft year it would have been about 4 pts less. So not bad but for a 1st round pick in his D+2 year you expect better. (off the top of my head I believe he had some injuries that year)
Then there's his whole development at the pro level which you just know is going to be suspect. So at 20 he's in the AHL, plays less then half a season (31 games) as I seem to recall he was injured at camp as the thought was he would have made the team out of camp I believe. In Laval he played a great defensive game but was terrible offensively and if I recall correctly he had almost no offense but then in the final week or two he started putting up points and then of course he gets called up just as he seemed to be finding his way.
He played 23 games in the NHL at 20 that year, then the next year he has injury problems and plays only 3 games in the AHL and 21 in the NHL. So after 2 pro seasons he has played just 34 games in the AHL (just a few games shy of exactly half a season) and 44 in the NHL. Then misses almost an entire year of development this season with just 13 games at the AHL, so now he's up to 47 AHL and 44 NHL games under his belt after 3 pro seasons. That's brutal for a guy that needed that time in the AHL to work on his offensive game and improving his mobility.