The problem is that Secrets are bad, so other cards were published to make them good, which means Secrets need to always be bad.
If you look at Secrets it seems obvious to me that they were balanced around the opponent having no idea what was going on. That is, if you actually play around them properly they end up being really poor spells that few people want to play. This isn't just true in constructed, but even in draft Secrets are generally poor cards you don't want in your deck.
Take Mirror Entity. If you know that this is a possibility, you play your cheap, bad minion first to trigger it. Or you play a minion that has a powerful Battlecry which the copy won't have access to. Or you just play spells that don't trigger it.
No matter how you handle it Mirror Entity ends up being weaker than most other cards. While there are circumstances where drawing it is amazing, there are far more circumstances where you desperately need to do something active and end up drawing dead into Mirror Entity, or you end up with a small, weak minion.
The solution would of course be to either buff Secrets or just produce better ones later, either of which could work. Unfortunately, Blizzard instead went the route of making Mad Scientist and Mysterious Challenger which make secrets awesome because you don't have to actually draw the bad Secret - it just happens for free.
Putting two Mad Scientists in your deck and 2-3 Secrets means that you mulligan away the secrets, play them when you have to, and the Mad Scientist gets you insane value. This has managed to make the Secrets playable, in the sense that they are overpowered when they pop out of a Scientist and weak when you actually draw them.
Here is the problem though - going forward Blizzard has to keep on making Secrets janky. If they actually make Secrets good on their own these Secret synergy cards will become even more bonkers than they already are. That will end up being a problem, as those synergy cards are already among the most complained about cards in the game just due to raw power level.
So what do you do in such a situation? I think going forward it is a problem to have Secrets rely almost entirely on overpowered synergy cards and be bad in and of themselves. Ideally I would like Secrets to be decent cards to play, or at least for some of them to be decent and others to occasionally wreck people because they weren't expecting the slightly subpar Secret. That sounds like a great environment!
In order for that to work the two powerful Secret enablers (Mad Scientist and Mysterious Challenger) need to be reined in. There are other Secret enablers but they are all pretty much garbage - if Secrets were just a bit better though they could see play. The two offensive ones though would both be strong enough to play if they cost 1 more mana, but both would suddenly become a lot less brutal. They would still be fantastic cards in decks that have Secrets, but there would actually be a reason to play lots of Secrets instead of just enough of them for your enablers to fetch for free.
As it is Blizzard has really put themselves into a bind where they can't make new Secrets that are worth playing and that seems like a really poor spot to be for a type of card that has so much promise. There is one ray of hope though, if they ignore my opinion on this: The more Secrets that exist, the better they become. It is really easy to play around 3 Secrets when only 1 is good, but really hard to play around 12 Secrets when 3 are good. The more there are, the better each becomes as they become more and more Secret and less and less "I know what that is!"