Its not for me. Personally I find building Lego therapeutic and I enjoy the Lego style of humour, so I'm not a Lego hater. A stand alone Lego racing game featuring proper Lego minifig style cars and not the weird looking Speed Champions (917 is perhaps the old decent looking car in the line), with building of the cars would have been epic, but mixing with Horizon as basically an aesthetics palette swap is just a bit meh. Horizon used to be about getting Forza Motorsport, taking it on the open road and making it a bit more relaxed and forgiving, now its like its trying to be a Fortnite Kart Racer. I preferred Horizon when it was about great roads and great cars, now its just drift, Ken Block, going 300mph, 1000ft jumps and product tie-ins and while the former is still there it seems to be taking a back seat.
I guess I hoped for more great driving roads, Fortune Island severely disappointed, I thought it a would be full of switch back roads but there was only one. Also despite being half the size of the main map, its road density was very low and uninteresting. Forza's maps never have any truly great driving roads and are too small and I always hope that's what the expansion will rectify, so I guess that is why I'm disappointed rather than it being Lego. Same story with the Hot Wheels expansion and this comes from someone who has a large collection of 1/64 die-cast and Lego.
The cars just don't work for me, I can't see why I'd want to drive the Lego version over the regular ones either.