So, when we look to ally our two pointy eared freaks, the first thing we'll pick up on from a mechanical standpoint is the massive disparity in philosophy, at least so far as transports are concerned. Both codices are renown for the speed and firepower of their transports. However, their durability differs vastly. That's part of what makes utilizing these two armies in tandem so awkward, any Eldar you bring will almost certainly not be fired upon, simply because your primary force, which contains your warlord, is so much less durable. This comes to the front of the problem. Why do you need these two armies to be battle brothers? They're even less cooperative than CSM and Daemons. Marines and Daemons might not be able to join each other's squads, but they can share rules. Which pretty much end at psychic powers. And much like that duo, these two don't really get too much from sharing those psychic powers, only these two have the problem even worse because of their reliance on, or at least strong synergy with, their respective transports. And remember, no matter how much I may love my half brother, my car is for immediate family only.
This helps us segue into some of my general thoughts on allies at present. Quite frankly, I'm a fan, at least of the idea. Coming from a background of a multitude of other games, I've seen radically different stances on who your army should be composed of and what rewards you reap for that. Some games, not unlike 5th edition, have demanded that you play a single force. Others have encouraged using a single faction through additional special rules that can be bought. Other still have allowed you to use differing factions and armies under certain circumstances, or by "buying" the right via points costs. And in almost every circumstance, it has been an interesting addition to the gameplay dynamic. Yes, yes, there's the obvious financial motivations for including the option, in that it allows players to buy more product slowly and build into another army incrementally, but most of my experience with allies has been much more positive than the most recent FoB debacle should suggest. While there are some quibbles with the allies matrix, largely revolving around Grey Knights and Tau, most of the time that I've seen allies used, though it presented a very strong list, I didn't necessarily feel that it was anything overpowering or damaging to the game on the whole. With this comes the admission that I haven't played against TauDar, but nevertheless, I don't know that the specific combination offers anything more damaging to the game than 3+ riptides or 6+ wave serpents do on their own.
Past that, we're only left to wonder what they do to keep us from outsourcing our points to the 4 or 5 codices which offer cheap as chips troops, especially those which have extremely forgiving allies matrices. Here unfortunately is where allies seem to fall apart and do more harm than good. For some codices, their internal balance was having cheap toys but expensive troops. The biggest example of all this is Marine armies, forbidding Chaos (they have cultists now). Marines get expensive troops, especially if they're using troop swaps. This usually means they'll either be shorthanded on troops, or miss out on some of their very potent non-troop selections. This really isn't a problem anymore though, given that these sorts of armies can now easily ally with either Coteaz led Grey Knights, Imperial Guard, or Tau, each of which offers various options for troops which are significantly cheaper than tactical marines. There's also the matter of considering allies working the other direction and those books utilizing marine-style non-troops to bulk out their killing potential.
So, I'll close with a question for us. Without making allies less interesting, how do we make the option to play out of a single codex more relevant or appealing?