Brewster was thrown into a team devoid of any confidence, losing every week and not creating any chances (and about to play City & Liverpool I believe). I think he may yet still come good but appreciate the view that in hindsight he wasn't/isn't worth the fee we paid for him, though I'd persevere when he's fit again.
McBurnie has been incredibly disappointing after a good start for us in the PL, with Brighton away a particular high point where he dominated some of the biggest and strongest centre halves in the league. Again I think some of it is down to lack of a run of games, I can't remember the last time he started 3 or 4 games in a row since the end of the first PL season when he scored a brace against Spurs and again made Dier and Sanchez look woeful at times. Not sure it will work for him here now though, think he needs a move and a fresh start to regain some confidence and form.
Berge came to us from Belgium and within 4 weeks the season stopped, he then got played in a position he didn't look comfortable with and suffered the terrible injury against Man U that still seems to be affecting him 12 months on. There's clearly an abundance of talent and skill with Sander but whether that's suited to the hustle and bustle of the Championship (or even the fast paced Premier League) I'm not sure.
I think in all 3 cases although none of them have performed anywhere near where we'd hoped they would, although all three have to shoulder some of the blame for that they've all been dealt a pretty rough hand at times too.
Forgot Olsen - Not sure with him. Lack of desire and motivation to play in the Championship at this stage of his career? Possibly coasting until his Roma contract expires, he certainly looks a lot better for Sweden.