I played two games at level 3 tonight (respectively an abysmal loss and a resounding win), and then read the rules for the remaining levels. Can confirm that level 6 sounds very difficult!
Level 4 is just a more difficult level 3 (with a slightly smaller ghost pool and slightly higher score thresholds to meet). I would have lost my second game if it was at level 4 as although I flew past the victory score in the end, I’d only barely reached the first of the score thresholds in time.
Level 5 makes graveyards much worse.
Level 6 tracks points independently for three different colours rather than as a single combined total, and you have to get all the colours past each of the scoring thresholds (which are lower than before, but then tripled in effect). Yikes. (Although it was evidentially much worse in the initial printing: unless you had exactly 3 players, you were playing these same rules with 4 or even 5 colours!! 3/4/5 players = 1 colour each; 2 players = 2 colours each; 1 player = 4 colours. They have since amended it to 3 colours regardless of player count – which is still hard.)
The manual also has a silly ambiguity about the victory threshold – if you base it on how most of the levels are played you would think the target was 100 per colour, but it’s actually 50 per colour (confirmed by the big tile with the level summaries).
On that note, the most awkward component/rule is probably the little target score marker that goes on the score track. Depending on the level you’ll have to reach it either once, twice, or three times, which I think is completely daft.
On levels 1 and 6 you’re supposed to put it on the 0/50 mark and reaching the marker means that you’ve won the game.
On levels 3 and 5 you’re also supposed to put it on the 0/50 mark and reaching the marker means that… you have to go around the board again.
On level 2 you put it at the 25 point mark, and reaching the marker means that you have to go around the board again.
On level 4 you put it at the 20 point mark, and reaching the marker means that you have to go around the board again… and reaching the marker that time means you that you have to go around the board again… and then you’ve won.
It’s a small thing, but so clunky and inconsistent, and it could have been solved easily with a score track that actually went to the highest score threshold, rather than making you loop around the traditional 50-point Carcassonne track repeatedly… or not repeatedly… as the case may be. 
.
Maybe they had a ton of the regular score boards printed already, and were determined to use them.
