Here's the issue.
The AI is bad, not just bad but terribad, It's honestly some of the most basic scripted toss I've ever seen, and I doubt it's actually better than BG1s AI. It has no tactical awareness, sense of preservation or ability to adapt to less than favorable circumstances.
For example, the AI will fireball your dragonbone fire pot chugged 2hander and do almost exactly 0 damage and no knockdown to you, but devestating it's allies, however the second it has the cooldown removed it will sling another fireball.
Archers have a very crude manner of attempting to stay at range, if you charge them they'll run a bit, but if you go stand round a corner they'll b-line straight towards you effectively running into melee range and awaiting death.
These are but a couple of examples of severe deficiency in the tactical aspect of the game. Essentially on equal footing, with half a brain you will never loose to the AI, it's too predictable and basic. Here in we come to the conundrum for Bioware. essentially in order to create a challenge the game has to have pre-positioned ambushes, with most of the brain work done without the game trying anything. This falls downs heavily however, if you don't enter the ambush the way the designer expects you too. Bioware attempt to solve this issue 2 ways, firstly they position your characters for you, removing any chance to counter the ambush regardless of how cliche it actually is, putting you in areas where scattshots will hit your entire party for high damage, melee surround you and ranged is in a good position above you out of easy access for counter melee (you're also often surrounded by traps that have no effect on the enemy). The other thing that happens is they do nothing, which makes supposedly difficult encounters a walk in the park as you sit in a defendable position shooting targets at range watching them come at you piecemeal as if you're playing an MMO.
This puts you in a difficult position as a player as you're forced to 'dumb down' your own brain and play into their traps. however this limits your combat options and dumbs down the game significantly, no longer can you use scenery to your advantage so the game essentially distills down to a DPS race, whoever can do the most damage fastest wins. This type of encounter is alright for the casual player as the tactic is essentially how they play, no offense meant.
The second technique is to just mob you with masses of troops, strength in numbers (or do both, put you in a dodgy situation and mob you). Here's the point whereby the casual player starts to suffer. A DPS race doesn't cut it by pure weight of arms, you can get wiped quickly and it seems daunting.
The issue is, even the mob warfare suffers at the hands of even a semi-competent RTS player, mobs can be walked into AoE disables interrupts which they eagerly and willingly charge idiotically into, the mob can be whittled down into nothingness as once in the killing field it has little ambition to preserve its life.
So to summaries, Bioware have essentially reaped what they've sown, but not how you think they are. Bioware are suffering from not advancing their combat AI. They've not made a hard game for those strategically minded but for those who enjoy RPGs but not RTSs they've made it at times nut twistingly difficult by essentially lazy game design. This isn't even bringing in crippling class balance issues, to which a RTS style player will use his companions strategically, while a RPG stylised player will choose his companions based on another set of criteria.
TL:DR = The AI or lack of it combined with class imbalance is the issue with the game difficulty, but the game is not challenging for anyone who's played RTSs.