What kind of momentum ETFs are you interested in? There are a few different kind and the performance as well as tracking error depend on details of the construction that differ wildly from one product to another. Take a look at this blog post: https://alphaarchitect.com/2017/07/24/m ... -and-size/
You'll find tons of material to read on momentum from Alpha Architect, Meb Faber and Gary Antonacci. My take away for momentum factor is that it is extremely powerful and has demonstrated consistent performance but is susceptible to gut wrenching crashes (relatively speaking i.e. tracking error) and devil's in the details of construction of momentum based products.
Just for disclosure, I invest in GMOM and VMOT (both US based) in addition to Gary Antonacci's Dual Momentum strategy. Let me know if you have any specific questions.
I've invested in Vanguard's value, momentum and liquidity based "smart factor ETFs". I haven't tracked them too closely but they have been profitable so I'm glad for that. It seems most ETF providers are coming up with some sort of smart beta ETFs to try and capture some sort of premium, but I don't believe there hasn't been enough time to conclude if it has been successful or not.
I just have a small portion invested in it, the main bulk of my portfolio is still in my core passive index investments, but having more diversity is seldom a bad thing. One advantage of these funds is that they have a lot of smaller companies that you wouldn't normally hold in a large market cap weighted fund.