If they are OK on 50Hz, then they will generally work correctly regardless of whether the supply is 110V with an earthed neutral, or 55-0-55 CT-E. However, there may be minor safety issues if they are equipped with a grounded or non-reversible cord that is intended to correctly identify line (hot) and neutral in the appliance (some US appliances have reversible 2-prong plugs where L & N could be either way round). In this case there is a possibility that single-pole fusing and/or switching is used that is supposed to be in the line conductor. With 55-0-55 both live conductors are lines, so a single-pole switch will no longer isolate the appliance and an internal SP fuse will not protect part of it against earth faults. Therefore, it is theoretically better to operate US equipment with an earthed neutral 110V supply, not 55-0-55 CT-E. Most transformers supplied with a US style outlet (NEMA 5-15 or 5-20) will have the supply neutral correctly connected through to the output (they are normally autotransformers) and these can be bought in all sizes.
Items that might not be happy on 50Hz would be those with induction motors (large appliances especially) which will run slow and hot, and large transformers which may also run a little warm, or hum more, or have more serious inrush.
