Are you talking about Q, W, or R?
Q: I would rather it be like Ahri's Q or Swain's E, not a moving zone that continuously reapplies a 60% slow for 2.5 seconds. It's not just Ekko that's a problem here; it's also Galio, Katarina, Talon (to a lesser degree), Xayah, Azir, AP Nasus, and Taliyah.
Abilities that deal front-loaded damage, then zone with the threat of additional damage are inherently problematic because the zoning requires no tradeoff (like Lux E) and you don't avoid all damage by leaving the zone (Maokai sapling, Illaoi, Heimer, etc). It's also a problem that these champs have incidental zoning power not factored into their power budgets, unlike champs designed for zone control (eg Zyra, GP barrels, Heimer, Teemo, Illaoi, Zilean, Cait, Mao, Cassio, Lux, Gragas, etc).
This is the exact problem with the old Yorick. His abilities dealt damage up front, then zoned with the penalty of additional damage from ghouls. It was most pronounced on him because the design was used on all of his abilities, but the design flaw doesn't become less flawed because it's on 1-2 abilities instead of 3.
W: Delayed and telegraphed are two different things. The delay is 3 seconds. The telegraph is .5 seconds, which is the amount of time you have to dodge an Arcane Comet or Xerath W (and shorter than Brand W and Zyra Q).
R: Yes. First reason is because the clone is buggy as fuck and appears and disappears so often (regardless of whether Ekko is actually nearby) that's it's unreliable to play around. Second reason is that because of Ekko's fucking insane move speed, the clone is often not actually visible.
Third reason is because playing against one Ekko requires the mindshare of tracking two moving units at all times. That's also part of the skill cap of playing him, but his players innately have more practice with the mechanic, making it more reflexive and lower mindshare. Playing against him is more demanding of attention than playing against most other champs.