Depends on the product.

I like the Fritz!box 7390 as it offers all major functions (Modem, router, 2Ghz + 5Ghz Wifi AP, VoiP server, data server, voicemail, etc...). It is the combo, my ISP provides (both as rental and standalone). No downtime in 36 months.

My other current ISP offers a combo device too. It's pretty horrid. It's a no name product, that regularly cuts out the connection for a few seconds. That is why I preferred the older solutions, where ISPs simply provided the most basic (but sturdy) modem available and customers went off to buy the router they preferred.

Offerall combo devices are not per se good or bad. Lock up, which devices your ISP and pick the one that gets good reviews and fits your needs.