Having units with duality is great. This means being able to hurt both tanks and infantry throughout the game. Units with high rates of fire and high(ish) strength are often great examples of units with duality. Their high rate of fire means they have lots of chances to destroy whatever they are shooting. Against infantry this important as cover saves are so abundant and against tanks this is important - not only in being able to actually roll on the damage chart but to ensure lasting damage happens and without Ap1, this is very important. Middling strength weapons (6-8) can shoot at both targets well. They have the strength to break through armor and the number of shots to make them count and they have the number of shots to inflict multiple wounds on infantry and the strength to do so reliably. This is great as your unit is always going to be effective no matter what.
This is where units like Dakka Preds, PR/MP Crisis Suits, Hydras, Psyfle/Riflemen, etc. or anything with multiple shooting profiles such as missile launchers shine - they always have some impact upon the game. This may sound great but knowing your target priority is really important with these type of units. This depends a lot upon your opponent and a lot upon your army.
The issue here is that this type of firepower can shoot both tanks and infantry. Great when there are only tanks or only infantry on the table - the unit is effective regardless. If there are both on the table though? You need to decide what to shoot and this depends a lot upon the unit, the opposing army, your army itself (what other units and firepower you have available) and the specific stage of the game. Far too many examples to cover so let's see if we can break it down a bit.
The higher strength guns are more effective against tanks. Whilst units like Riflemen/Hydras are pretty good at causing 3-4 wounds on infantry a turn, their best value is in downing low AV tanks and since transports are so common in 5th edition, well they often have a lot of targets in this regard. The lower spectrum of duality purpose firepower doesn't have this luxury against tanks. S5 in particular can only penetrate AV10 so it's a pretty easy choice to shoot this type of firepower at infantry unless there are some AV10 tanks around - or you really need to shake AV11. S6 though...well it's quite decent at damaging AV10 and passable against AV11 but importantly has reached the spot where it wounds most infantry in the game on a 2+. Again, when you start going higher it's an easier choice to shoot tanks but S6...well against some armies like Imperial Guard or Mechdar you just don't have a lot of chance to actually damage the tank and even against AV11, your chances to penetrate are pretty low for every shot.
This generally relegates S6 to shooting at infantry for the majority of the game and is often very effective at taking out small Marine squads. Marines don't like making a bunch of saves and S6 doesn't care about their better than average toughness nor that they have 3+ saves. It just forces MEQ to make a bunch of saves and sooner or later they are going to fail. This is a major premise of Eldar Mech, Grey Knight Psybacks, Chimeras, etc. - it can damage AV11 in a pinch but against MSU Marines, once those AV11 boxes start popping you get to put a bunch of wounds on Marines very reliably. Armies which start Marines outside of transports such as Long Fangs, Devastators, combat squadded Tacticals, etc.? Well they are all prime targets for that S6 shooting early on particularly since their threat range is greater than melta units inside Rhinos/Razorbacks and higher strength firepower such as Lascannons, Missile Launchers, Autocannons or your own melta is better off taking these targets down. You generate maximum damage and have maximum damage potential this way.
Now again, this is only a specific situation and a lot of factors come into play but when you have units which are effective against both tanks and infantry, you really need to know your target priority and how this evolves over the game. Having Psybacks or Chimeras shoot Long Fangs in the early turns of a game is great. They are more likely to cause damage than against Rhinos or Razorbacks and the melta units in said tranports shouldn't be in optimal range as of yet. As the game unfolds however, being able to stop those melta units in the transports becomes more important than shooting down missile units (which are likely to be weaker at this point) as meltaguns and follow-up assaults have more damage potential than missiles at range.
Again, you must understand both your army and your opponent's and how this affects the target priority of units with duality. Having units with duality is great as it gives you a lot of options but that also opens up more avenues for mistakes. Specialists units don't really have this conundrum - they are either good at what they shoot at (and therefore you aim to shoot at this particular target) or they are inefficient. This is where units with duality win out but only if you make the right choices as the game evolves. Not doing this will hinder your army and whilst it may work occasionally (i.e. you get lucky and your Psybacks blow up Rhinos like a knife through hot butter), it hurts your army.
