People didn't hate the aesthetics. They hated the fact you could crush him with minimal effort, yet he still survived thanks to plot armor and proceeded to mock you as "weak".
This is definitely one of the reasons I disliked it. I never felt like I was getting beaten, I felt like the story just FORCED me to get beaten. This is likely largely due to the clashing of gameplay elements. Mass Effect is clearly a shooter and then they throw in this space samurai. The entire fight, for me, was just shooting his plotarmor biotic recharging bubble and staying out of cover for the air support that conveniently leaves the moment Leng gets his space juice back.
Know what was a more difficult boss battle? Citadel DLC. The final battle with clone Shepard was actually a bit of a challenge depending on your difficulty setting. He was running around, cloaking (depends on class I know) and managed to trap me into a CQC scene many times. I actually had to use medigel for a change.
No such challenge was ever had with Leng, imo. Not even whe dealing with the phantom squad
Another thing I disliked was the complete loss of potential. Leng was a bit of a badass in the novels. They could had done so much more with the character. They could have actually made me hate the character instead of forcing on this hamfisted attempt of creating this arch nemesis clown.
I felt like the character deserved to be far more challenging than he was. Maybe also have him take your LI or one of your squad hostage or something. For a racist he sure had a tame tongue considering my Shepard was cruising around with two aliens all the time.