Civ VI is a great game, it's already got great mods (so "use VP" isn't really an argument; AI+ and Civ Flavour Deity did a good job of fixing AI issues in the past and probably will do so again once they're updated and if you do dislike the UI you can use CQUI which is apparently (don't ask me why; I don't like it) much better). And that's basically the only two reasons why people dislike Civ VI.
As for Civ V, there's just a lot of fundamental errors in that game. To balance expansion (instead of making it a mindless "more is better", something all Civ titles struggle with in one way or another) it gives straight up penalties, which are also very visible and because of that very annoying. A major source of income is connections between cities, but you get those by building roads... which cost maintenance, making it a mathematical exercise to find out if you should build a road. All your empire happiness is calculated in a single number. Whenever that dives below 0 for one reason or another (every time a city grows, for example) all your cities have 75% reduced growth, making for a very staggered growing of your cities. The World Congress is extremely tedious to work with, as typically you just end up banning random luxeries. City states are controlled by throwing money at them. Diplomacy victory is basically economic victory, as you win a diplomatic victory by getting votes in the World Congress, which you get from controlling city states.