ISFPs have nothing that I dislike on any kind of regular basis. Their flaws seem to be of the type that are detrimental to them, not others. Imo they are the type I'd probably choose to be, although of course I understand I am me and I am happy with myself etc...
They are gentle and caring and never want to hurt people. Their problem is they are too trusting and can be overly sensitive. They need well-intentioned NTs (who understand their nature and can talk their language!) to keep them out of trouble and to explain things rationally so they don't get too hurt! (just kidding, although it helps...) Seriously, my sister is one and stuff that we wouldn't even notice can hurt her very much.
I think it's a very good complement to the NTP type; we learn a lot from them because their P makes them just as laid back and interesting in sitting around and talking about all kinds of things, yet they are so different so say things we would NEVER EVER think of. Together, and done properly, it is really the whole is greater than the sum of the parts type of situation.
Edit: so to answer the original question, based on the people in my life that I know well, so not exactly scientific.
SP: can have a lot of trouble thinking ahead (no strategy)
SJ: can have a lot of trouble adapting to quick changes (inertia)
NF: like to help people so much they can be taken advantage of (too subjective)
NT: can be completely oblivious to the fact that feelings matter (too objective)