Ok here are my examples: we are playing "Undermountain of the mad mage".
My first character seemed really underpowered even though i don't think she was. Whenever I create a character I like to use every tool at my disposal to try to overcome problems. However, we weren't even allowed to have any equipment at all, and we were very limited on money despite starting at 5th level. So when we did come across a shop we weren't able to purchase equipment, because even though we were dungeon delving we weren't been able to find any treasure. So when the chance came to do something reckless for treasure I felt that I had to take chance on it. Even though it should have been a more fair gamble. We were passing a Drow elf and a Chimera, all the party was walking past I was bringing up the rear and decided to cast darkness on my sword the elf considered that an attack and that started combat. While my party was engaged I decided to drop the sword. It still had darkness cast on it, so I could start to act as a buffer between me, the enemies and the treasure. It took two rounds for me to do this mysteriously.
Then when my party ran from the enemies the elf was able to magically find my sword in the darkness and cover it up and attack me all in 1 round. I was killed.
So I rolled up a new character and this time I went for power gaming. Eighth level Drow Elf Thief Assassin (3), Ranger Gloom Stalker (5). I never have been able to use my Umbral Sight feature because for some reason it's never dark during a fight. Lately it's even been sunny in the Undermountain (keep in mind I'm a Drow!).
Also it seems every Drow that's an Npc can magically see through magical darkness. No matter what I roll on initiative ( mind you I can add my wisdom modifier to my rolls due to the Dread Ambusher feature) my DM always seems to roll higher, making the Assassinate feature completely useless.
I've been targeted with ridiculous rolls which always beat my AC ( I've only have not been hit twice) which I wouldn't mind but there are literally people in the group that never seem to get hit with AC's around 14-16. However I always choose to lead the party so I accept to be targeted more.
My biggest issue was our last encounter: my Passive Perception is 25 my active Perception is 12, I have Darkvision ( even though it's useless due to the fact it's never dark, ) out to 150 feet, and I have Blind-fighting out to 10 feet. Surprise round: one of our party is surprised by a Cloaker that wraps around his head. Next round I attack it without Assassinate, attacking 3 times and wasting two attacks hitting illusions. The last attack does pitiful damage (9pts) however I tell the DM I actively search around everywhere to make sure it's just 1 Cloaker. On the very next round still before I can get my turn another cave opening (that wasn't shown on the map originally) appears and a Cloaker wraps around my head and attacks. Am I just overthinking it or is my DM being unfair!?