Driving restrictionはなさそうです

Driving restrictionsはなくA cross-border feeさえ払えば問題ないと思います。
c/f
Hertz vehicles cannot be driven into and/or dropped off in any African, Asian, Middle-East countries, any island, nor across the borders of or beyond the following countries: Albania, Armenia, Azerbaijan, Belarus, Estonia, Finland, Georgia, Ireland, Kyrgyzstan, Latvia, Lithuania, Moldova, Russia, Turkey, and Ukraine. (As of 2018-9)
余り経験ありませんが､サラエボ－Opuzen(モスタル-R424/E65-Opuzen)の国境越えはスムース(10分程度)、飛び地のOpuzen-A8-Dubrovnik)の国境越えは簡単、Dubrovinik-A8-Kotorの国境越えには時間が掛かります(1h位　As of 2012-5)。