Having now played three Skacky Thief Gold FMs, two of which had negative comments that originally made me question the hype, I can confidently say that comments under Fan Missions like those can be safely ignored.
I've been playing through (but have yet to complete) "Endless Rain", the direct sequel to "Between These Dark Walls", and I can safely say that every good thing said about it is deserved. The map is layered and nuanced, and exploration is challenging and rewarding.
If I was being charitable, I'd say the criticism is directed towards individual paths through the mission.... while the majority of the map follows the usual "Hub/Spoke" open world pattern you'd see in a City Outdoor type map, individual branches off the Hub do tend to be very linear... the basement only has one path up to the first floor, the vent there is the only way up to the second, etc.
But even if each of these spokes are very linear, they rarely ever dead-end, and typically connect back to the street area hub in fun ways. I started out robbing a place by picking the lock on a cellar door, but exited it from the highest balcony, parkouring over gargoyles, then balancing along a drain pipe to hop onto the balcony of a neighboring building.
God bless every DromEd mission maker, they've been breathing life into a fun game for damn near three decades now.