I have playtested all 3 pacts at each tier and various levels, and I have played a fiend patron tome pact warlock from 1st thru 20th levels. I typically stick with lore bards as a personal favourite but I prefer warlocks to every other major spell casting class aside from bards.
The flavour and fun factors are excellent. Those are always my priority.
Mechanically, early in levels the short rest gives more slots per day than other low level casters. The party cannot simply take 7 more hours because the benefits of a long rest can only be taken once per 24 hour period, meaning those other 7 hours do nothing but turn it into an 8 hour short break, normally. Once the other spell casters are catching up in daily slots, the smart choice is in having spent an invocation or two on at-will SLA's that would normally also cost other spell casters slots for which the warlock wouldn't need to spend. Later, spell slots do increase, more and better SLA invocations become available, and arcanum are gained. Given that 2 short rests gets up to 12 5th-level slots on two short rests it becomes challenging for other casters when the bulk of their spells are cast below 5th-level slots instead of equal or above.
It's really easy to make a competitive magician out of a warlock. The only difference between chain and tome is the better familiars versus more cantrips, plus the assumed ritual book instead of the chain restricted invocations.
The idea that slots need to be used on hex is a bad assumption. The option does not need to be used simply because it exists even if it's a warlock unique spell. Eldritch blast already hit top damage for cantrip options beyond most selections available to other spell casters, and one invocation at 2nd level puts even farther ahead of almost every cantrip option available. That's why it's a popular splash choice. Hex should be a choice when it makes sense, but hanging on to that choice while ignoring other options is a player issue, not a mechanical issue.
What's ironic is part of the complaint stemmed from spamming eldritch blast but that's exactly what other characters taking the same option via warlock splash are also doing, and they also use their concentration to cover it, but they also gain access to higher level abilities with a 2-level delay so the warlock gains higher level spells available sooner in doing the same thing as those other classes taking the splash. It's even more ironic that pretty much all low level spell casters are spamming cantrips a lot but the warlock clearly excels in that area.
Blade pact warlocks are a bit different, but medium armor is available with a feat or building from a dwarf or via a splash, or the character can use DEX as the primary combat ability score. In my experience, they take more careful planning but also typically out damage eldritch spammers most of the time with those focused builds. The problem with cantrips in general is that most cantrip options don't do ability modifier damage once and the options in other classes that do add that damage tend to be once. Eldritch blast is the exception to this rule and still available as a ranged option to blade pact warlocks. Eldritch blast isn't generally worth looking at over weapons until 11th level on that 3rd blast but the 12th-level blade invocation makes the difference back up, and feats can keep the blade warlock ahead even with the 4th blast at the highest levels. Without feats the weapon user would need to rely on poison but doing so still gives weapon use the advantage even without feats.
The damage resistance available to fiend patron warlocks is another important consideration and one I would recommend on the blade warlock.
Blade warlocks are effective but take more advanced planning.
Overall, I find the concerns raised in the OP unwarranted. In my experience, taking advantage of at will SLA's in the invocations is definitely worth while.
The final comment that I would add is that the "5mwd" is a player created issue that I see in discussions but not in gameplay. Players manage their resources instead of blowing their spells. The first thing that springs to mind when the subject does come up is that one player is withdrawing from active play while everyone else continue to play and have fun while continuing to acquire experience and treasure. It is not that the DM forces the players to continue playing; it is that the players are roleplaying instead of gaming the mechanics and don't show a rest at will sense of entitlement. Short and long rest opportunities occur at adventure points naturally.
