Based off my experimentation, and therefore subject to personal opinion (individual results may vary):
Mekanism opens by trivializing steel production from other mods, namely RailCraft and Immersive Engineering. I know TE Induction Smelter can make it too, but because Mek relies so heavily on steel so early in the tech tree, it allows steel to be made quickly and automated relatively early. This could possibly be addressed by removing the steel recipe from the Metallurgic Infuser and forcing steel to be produced via other methods, which would slow the tech tree down considerably as you waited for Blast Furnaces to keep up with your demand.
Next is the Jetpack. Now, don't get me wrong, I love this thing, but flight trivializes many aspects of Minecraft challenge, and Mek offers flight very early and cheaply (what with fuel being a byproduct of your early ore processing). This alone alters the landscape of any pack, as most mods save flight for a mid-late game reward. This could be addressed with a recipe change to the Jetpack. Requiring Reinforced Alloy (or even Atomic Alloy), Vibrant Alloy, or Enderium Ingots in the place of the default Tin Ingots would help to better place it in most packs, although this may hinder it's appeal by doing so.
Power generation is something Mek is very good at, and for good reason. Once you get your 5x processing line set up and upgraded, it eats some serious juice. However, the scale is not proportionate to the scale of power generation in most other mainstream mods. For instance, a single Ethylene production line can create enough power to outclass many late game power options from other mods, while using fully sustainable renewable fuel. Even before that, a single Wind Turbine can supply fuel free power to farms and outposts at a very manageable price. Fixing this requires either substantial recipe tweaks and/or deep config adjustments to power generation. Unfortunately this means people will often just spam more to make up the difference, as the 5x processing is still power hungry (as I believe it should be).
The Digital Miner is wonderful, but amazingly powerful compared to other mining options available. It is usually disabled altogether when mixed with other mods, but it's worth mentioning.
The 5x ore processing is generally viewed as being overpowered, but if power is reduced at generation, it becomes a late game luxury rather than a mid game turning point. This does encourage people to spam power gen, however, which can be hard on servers. Left with default power generation, the 5x system quickly trivializes resource acquisition methods.
Mek machines cause a great many lighting updates. This is actually more of a personal bitch than anything, as my computer is subpar, but I think the point is valid. If special precautions aren't taken to stop it from happening, many of the Mek machines will stutter as the production line flows (making a single item, then shutting down or enough gas to run the next operation, for instance). When these machines are operating, they light up slightly, then go dim again when shut down. This stuttering of lighting updates can massacre my FPS in production areas. Taking the time to set up automatic shutoffs and limits can make this much better (less frequent updates due to batch building), but many people won't take the time/effort to do it. What's worse is building the entire setup and then finding out what the cause is (like I did), since fixing it in an area with single digit FPS can be quite frustrating.
The last time I played with it, Mek pipes and cables didn't play well with other mods, nor did those mods play well in return. It made mixed mod systems much more difficult to get running properly. This may already have been addressed, as this was at the beginning of the 1.10 window, so I leave it as a minor concern, but it was a concern enough to mention.
None of this is meant to dissuade anyone from using Mekanism, nor to discourage it's inclusion in future FTB packs. If you haven't already tried Mek, it's worth a highly recommended play through. This was meant more to showcase some of the inconsistencies between Mekanism and the field at large which tends to leave Me on the outside looking in. All of them can be addressed and overcome, given the time, drive, and proper testing. In the end, I do hope to see Mek in more packs in the future, as the mod itself is very well done and worth a bit more spotlight.
