I entered Ukraine on my motorcycle last year from Slovakia, had no problems of any kind. There was a bit of a line-up, the border staff carefully checked the ownership and insurance documents for my motorcycle (I think this was in my own best interest - it appeared that they were checking to make sure I had not stolen the bike in Europe), and then they stamped me in and I was on my way. Total time about 30 minutes, very polite and professional border guards.
There was quite a line-up at the border to go from Slovakia into Ukraine - I joined the end of the line, but as soon as the locals (daily commuters) recognized that I was a visitor, as soon as they saw that I had Canadian plates on my bike, they all waved me up to the front of the line, which was a very nice courtesy.
Leaving Ukraine into Romania, no problems at all. The Romanian border staff spoke English, they were very friendly, just took a quick look at my passport, stamped it, and said "Welcome to Romania". About 30 seconds total.
FWIW - I was not impressed with Ukraine. It's a rough country. Romania, on the other hand, was delightful.