2)
Hmm.. well I don't use many Stroke it cards. My most used set has about 5 Stroke it cards per 60 other cards.
But general imbalance of card types is a problem if one uses few of the general cards in my opinion.
For the card types I use few I generally either edits the cards to include something that ends the effect except another card of the same type or just make a general rule for myself that those card types also always end through some other condition. Else some effects can become quite unpleasant, because they usually aren't intended to last for half a tease or more. (or they just get boring)
1) If I use them cards that simply do +2 or -1 stroke rate are rather boring, even though I also have one of each of those. Cards like
this are already more exiting (and can be nicely automated with the teaseviewer). Aside from that change of stroke position can also be used for more uncomfortable positions. Positions that are more straining for the body can go closer in the direction of a rough mistress, especially if held over longer periods time, so that can also be used to spice things up.