Hattrick Soccer Manager is one of the longest running and most successful online soccer managers - established in Sweden in 1997 it is now almost 20 years old. In 2009 Hattrick was one of two (next to Manager Zone) such games with largest number of users - around 1 million, although exact numbers are vague and hard to confirm. Unfortunately the glory days for Hattrick are long gone - in 2012 the original creators sold their game at its prime to a British company that within 1 year runned the game down and soon later sold it back to Swedes.
Hattrick from the beginning was built as massive multiplayer game, which meant some sacrifices, f.e. only one transfer option is available (auctions) since ot would be impossible to spot the cheating parties, the rotation of users was always high, the divisions structure was a huge pyramid where there is no way to guess where we will end after promotion or relegation, etc. Those factors made Hattrick a very superficial game that lacks depth when it comes to player parameters (only 8 and they determine player position rather than describing his skills), player statistics, management options and so on. In general the game is based on very simple engine that in first step compares the abilities of midfield players for both sides - whoever has advantage there will have more chances to score goals. In theory it seems fair, but in practice it leads to lots and lots of games in which one side is dominating the match and results like 5-0, 8-0 are not something uncommon.
Another problem is with the game development - there is none. For years only tiny details were changed in the game and it does not seem like the creators really want to improve Hattrick anymore. They have introduced youth teams that were a novelty, but proved to source of huge amount of good players that flood the transfer market and with each season it is easier to get better players much cheaper. On the other hand the training system is very simple and very predictable (f.e. 17-year-old player will improve his skills within 7-8 weeks, 18-year-old in 8 or 9, 19-year old in 9-12, and so on), which means that there is any point in buying and training only very young players.
The league system has tiny flaws that made huge problems with time - the differences between each level of divisions are huge and since the number of fans coming to stadium are linked to current season results sometimes it is far better for club not to get promoted since one level higher they will most probably lose money instead of gaining them. The simple example: if you take a look at the national teams most of their players do not even play in the top level clubs, most of them play in 4th, 5th or even 6th division.
Overall Hattrick was a brilliant idea that gathered a huge interest from fans, but simple solutions to transfers, training, youth players, forums, etc. that worked fine at the beginning were not changed with time and a lot of elements in Hattrick right now are really obsolete. Of course there is still a large number of devoted fans of HT, but for few years now the number of players have been decreasing and without a large scale revolution in whole system the game will most probably die (especially since more and more options are available in premium only and lately the price of the premium account was almost doubled).