Basing on my experience of arriving in C from London and leaving from A, 90 minutes is very tight. Remember, United ask you to be at the gate 30 minutes before departure for international flights; even if they cut some slack I guess in practice this gives you 75 minutes at most to actually get from arriving gate to departing gate.
There are two main areas of uncertainty: immigration line, and re-clearing security on entry to Terminal C. Other delays are Bag collection, custom clearance, bag recheck and transit B to C.
Immigration line: this can be much longer if you are a non-US citizen. I am pleased when I clear immigration in under 40 minutes after disembarking.
Bag Collection, Customs, Recheck - not usually much of a delay, even so 10 minutes here.
Transit B to C: the train itself is quick, but you add a few minutes walking and waiting. another 10 minutes. Also sometimes on Sundays or in inclement weather they operate a reduced service on the train, so there can be additional delays.
C security. the published wait times show that 10-20 minute delays are common at terminal C.
Walk to gate may be quite long at terminal C, allow 10 minutes
That gives gives an estimated transit time of 80-90 minutes. Personally I never take into-USA transfer times of less than 120 minutes. I realise (hope) that the airline will sort things out if I miss my connection, but if that means a delay of a day, which it might well, this is not good.
Edited to add: in answers to my question about connection times one other factor is explained: The airlines may set their connection times taking into account statistics for early arrivals: If they know that on average a certain arriving flight is 20 minutes early then the may allow a connection time that seems quite aggressive. The claim is that the airlines do set realistic connection times.
My main bugbear remains the queues at immigration, especially for those folks who cannot use the Global Entry machines - I find these delays to be very unpredictable, and sometimes very long, hence I will still be very conservative when booking my own flights.