Personally, I believe Degroot Keep is flawed because of the existence of the Huntsman, making all of the classes hard-countered by a semi-competent sniper using the Huntsman. The Huntsman is even more powerful with the fact that there are places to light your Huntsman arrows on fire to essentially one-shot 4 of the 9 classes. The choke points are horrible, especially on Control Point C. The singular entrance to the RED base makes for Huntsman spam as if it were Overwatch. I know that the game mode is supposed to be for fun, but it is obvious that the majority do not like the map when half the server leaves once the map loads onto Degroot. The server dies because the people who do not want to play the map join on a different server. Yes, RTV exists, but it requires ~75% of the players to Rock the Vote that is is uncommon to happen. If people want to play Degroot Keep, they can simply boot up Live TF2 that also has the demoknight.
In all of the times that I have nominated this map, and I can look through my hundreds of hours of recordings, I can count the number of times that the server has died on one hand. If the server dies, it is more likely to happen on a map such as Dustbowl, Junction, or any other map with a long, painful objective. Furthermore, there are a number of times that the server decides to RTV past the first 10 or so minutes, as occasionally, there is no easy way to balance the particular teams that are playing.
The primary reason that I quite like Degroot is that it is the only map that feels like a true adrenaline rush, with nothing at distance to worry about but some fairly inaccurate huntsman arrows that are fairly easy to avoid (again, I have played very few matches where the huntsman is unbeatable), and one truly monumental achievement to storm the castle, as it is very difficult to turn the tide once that has happened, further increasing pressure to keep BLU out.
However, what probably frustrates most, but delights me, is that there is no one winning strategy, leading to potentially endless replayability, and looking to capitalize on the other team's mistakes. Furthermore, there is no demoknight crutch waiting to launch over the walls and steamroll the team with speed, reach, and health, which frustrated me the few times I have played it on live TF2.
I know that I stand as either one of few, or as the only one that holds this opinion (and rather infamous for it, I admit), and I am nearly certain that it is too little, too late, but I would like to leave one last thought on the table:
If it were really such a bad map as it stands, why do the servers vote for it, even when the largest proponent (me) is not playing? There have been a few occasions that I will log in after a couple of days off, and I can join a match of DeGroot on a VF4 server, and most seem to enjoy it. Rather, it is the few who are experienced or skilled that are frustrated that they cannot play their typical tactics.