Perscienter: No proper discount end dates (like GOG).
Giltonnam: When they have games marked at 75% off it normally means it's only available at that price for 1 day. Just like Spec Ops: the Line and M&B Collection. (Visibly marked on the top left of the frame). I haven't found anything confusing with their site and have recommended it to others for the great prices.
Perscienter: Unreliable payment.
Giltonnam: I've never had any problems with paying with PayPal. I've bought several games over the course of months and never had any issues whatsoever. Did you have an issue with your Paypal account in general? That's the only cause I can think of with my experiences.
Perscienter: Data-hungry.
Giltonnam: As far as being "Data Hungry" by wanting your contact/address, EVERY SINGLE online store I've come across requires you address and/or info. Amazon, Steam, GoG (though much less specific iirc?), As far as i know this is for legal purposes should something happen. To be angry at this in current times is really just silly imo.
Perscienter: Shady discount conditions.
Giltonnam: Nothing about the discount conditions are shady. Normally things that aren't eligible for discount are visibly marked. Maybe you tried to use the voucher on a special marked promotion and didn't notice?
Perscienter: After that you would still have to deal with Capsule, their own software, but I won't find out how bad that is.
Giltonnam: I haven't tried out capsule yet, haven't bought any games that require capsule. So can't comment on this.
If you're having issues with GMG deals (on if they work with vouchers/when they end) then I suggest checking out
http://www.reddit.com/r/GameDeals/ . They normally state the the price, if it works with the current discount code, and when it ends.
Edit: Just saw your voucher image. That's really odd, no clue why that is. O.o
Edit 2: Maybe because it can be traded back in. "Excludes titles on Cash Back or
GMG Credit promotions."
Do you work for Green Man Gaming or have friends or family who work for Green Man Gaming? I'm guessing the answer is yes.