I used to play in tournaments when I was younger, and I did very well in the last tournament I played in with DE.
However, I think the success of a DE player is more determined on the table, mission and match-up than any other race. Kill Points, for example, is something that DE just inherently suck at, since most of our units are fragile as crap and the transports used to carry them are basically made out of paper. The table also matters a lot because if you're playing on an open field vs. something like IG, you should basically gg out and not bother playing. You need 25% with a good variety of line of sight blockers to make a substantial difference in that matchup. Lastly, the matchup itself: A Venomspam army will 20-0 almost any Tyranid army, really. Then you look at the same list and you draw Mechdar with 6x Wave Serpents and WWs and you basically gg out because there's nothing you can do. You have 1-2 rounds before your entire army dies essentially.
This is why tournaments are hit and miss, and why there's some DE players that place high, and some that don't, even if they're the same caliber player.
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