I can't stand these guys who don't know shit about Achilles locks so they say they suck, aren't high percentage, can just tough them out, if the guy is taller he'll catch you too and then you'll be in trouble. Simplily put, if you do not know how to perform correct leglocks. DO NOT try them.
Most people incorrectly position their forearm and have an incorrect grip, that is reason number one most people suck at them and think they are ineffective. Also people tend to incorrectly go for the lock and give the other guy their leg, which will not happen if you correctly position your feet and legs.
One example of how an achililes is effective if done correct, and why it is a horrible move to try if you can't. My first Amateur MMA fight my opponent backed out (I had already gone to the trouble of cutting to 189, the event had screwed up weight classes) so I wanted a fight, I found one guy who did not have a a fight. He weighed in at 239, but confident I could kick his ass I took it. I rushed in with my striking too quickly, left myself open for a takedown from the adreneline dump I just wanted to knock his head off. So as I was flying through the air I though "once this hits the ground he's f*cked" Anyhow, about 10seconds on the ground I had him in an armbar, felt something in his arm pop/snap but he didn't tap. After a few near submissions and him never coming close to passing my guard. I stopped striking so he would base up, I then opened my guard and wondered if he would be stupid enough to try a leglock on me. So I left me left leg out and since this guy never learned how to pass a good guard (apparently never learned leglocks correctly either) he fell back to go for an achilies. Of course I immediatly hooked his leg as he fell back and turned my foot to defend. This guy thought we were trading locks and I was in danger even though he had nothing about so he didn't tap immediately about 2 and a half to 3 seconds of me having locked in my achilies I felt a crack in his foot heard a scream then he tapped. And let me tell you the look on his face in the video the moment before he tapped was priceless. Broke his ankle. Snap Crackle Pop.
Moral of the story, do not try leglocks if you they are not nearly 100% for you and if you haven't had success always getting a quick tap.
Some people just don't pick them up, I generally learned my leglocks from watching some fight tapes and decided to "try that technique" and somehow I did it correctly and have loved leglocks since then. Of course throughout the years I've refined my technique, trained with some guys who know leglocks and improved my over all technique. But the one reason I ever got good with the achilies was because the first thing I got right was the position on the ankle and the position I made with my forearm bone and my hand grip.
I like giving people good grappling advice on the net. Passing the guard, side control, knee mount. The key basics but for some reason leglocks are the one thing I really only like explaining to a few people I train with in person. I would recommend however taking some Gokor or Erik Paulson seminars, finding some guys who are really good with leglocks to train with because correctly knowing Ankle Locks, HeelHooks, Kneebars, ect.. Will improve your game tons and imho are one of the most complicated submissions which is one reason people always try to claim they take no technique or they are "cheap" simpily because the person ego cannot take the fact they have no idea how to perform the techniques.