I just played a Battlesim this weekend with mine. What specifically were you worried about? The obvious thing is that if the MilSim permit mid-caps that means you’re at a massive ammo disadvantage being stuck at 35 rounds and of course you need to consider whether or not you need to load gas on your person if it’s a 24/48 hour MilSim as that’s probably preferably to carrying a dozen or more £50 and heavy mags. Of course there is a mod you can do the mags to get 3+ fills out of them to alleviate this, but it still might not be enough depending on playstlye.
On the ammo front, it’s less of a problem than at a skirmish that permit hi-caps so that point’s kind of irrelevant. In terms of ammo expenditure, I personally go through around 200 rounds in 6 hours as I fire exclusively on semi but that’s obviously down to personal play style more than anything else.
Unless you have specific concerns/questions I’d say go for it. As with any gun you need to be mindful of it’s strengths and limitations but the MWS doesn’t pose any notable disadvantage I’d say beyond carrying gas, which is a bigger, more general question in the GBB versus aeg discussion rather than being unique to the MWS anyway.
If you’re not sure whether it’s your mag or your loading method that’s causing the issues I’d suggest you use the pole and plunger included with your MWS just to test the mags out. I actually still use that even at games as I haven’t found a speed loader that works yet.