The Adeptus Mechanicus, as a whole organization, remains deeply conservative in its approach to technology. The vast majority of the AdMech still adhere to ritualistic maintenance of machines and most tech-priests do not innovate in any significant ways. There are exceptions to this rule, however. For example, the Xenarites are a radical and secretive sect within the Adeptus Mechanicus that focuses on studying and using alien technology. This is obviously considered heretical to varying degrees, but as long as they don't flaunt it openly, the Inquisition seems willing to turn a blind eye (for the most part).

You can innovate, experiment, and even introduce new technology within the Mechanicus, but only if you possess enough clout and influence. Even then, you are up against 10,000 years of tradition. The Mechanicus still hoards technology and conducts searches for lost STCs with explorator fleets. In fact, high-ranking members of the Mechanicus often keep rare tech for themselves instead of sharing it with the wider Imperium or even other Forge Worlds.

The Mechanicus does have superior tech compared to most of the Imperial Guard/Astra Militarum, but it's not surprising that they keep the best weaponry and equipment for themselves. Most of that tech is not "new" but rather 10,000 years old and has been produced in the same manner, with only minor adaptations, since the year 30k. The really exceptional stuff, like various types of robots, only exists in highly limited capacity and is usually only brought out for truly important battles or the direct defense of the Forge World.

I'm not saying you're wrong about the old Mechanicus lore being different, but from a player's perspective, I prefer the new lore. Many old elements are still present, and the Mechanicus remains an organization that is superstitious and often, due to dogma, less efficient than it could be.

In a way the new lore makes it easier to make your Admech army your own flavour of monstrous theocratic cyborgs and Im always in favour of giving players more options to make an army "your" dudes.